UNIVERSITY  OF  CALIFORNIA 
SAN  FRANCISCO  LIBRARY 


GEOLOGY 


BY 

Sir   Archibald  |  Geikie,    F.  R.  S. 

D.SC.  CAMB.,  DUB.;  LL.D.  EDIN.,  ST.  AND. 

CORRESPONDENT    OF    THE    INSTITUTE    OF    FRANCE, 

DIRECTOR-GENERAL    OF    THE    GEOLOGICAL    SURVEY    OF    THE    UNITED    KINGDOM,    AND 

DIRECTOR    OF    THE    MUSEUM    OF    PRACTICAL    GEOLOGY,    LONDON}    FORMERLY 

MURCHISON    PROFESSOR    OF    GEOLOGY    AND    MINERALOGY    IN 

THE    UNIVERSITY    OF    EDINBURGH. 


WITH    MANY    ILLUSTRATIONS 

E1 


c-trittoit  He  iiurc 


CoRege 


NEW    YORK 

J.  A.  HILL  AND  COMPANY 

1904 


CONTENTS. 

CHAPTER  I. 

Page 

INTBODUCTOBY 1 

PART  I. 
THE  MATERIALS  FOR  THE  HISTORY  OF  THE  EARTH. 

CHAPTER  II. 

INFLUENCE  OF  THE  ATMOSPHEBE  IN  THE  CHANGES  OF  THE  EABTH'S 
SUBFACE    10 

CHAPTER  III. 

INFLUENCE  OF  RUNNING  WATER  IN  GEOLOGICAL  CHANGES,  AND  HOW 
IT  is  RECOBDED  24 

CHAPTER  IV. 
MEMORIALS  LEFT  BY  LAKES 44 

CHAPTER  V. 
How  SPBINGS  LEAVE  THEIB  MABK  IN  GEOLOGICAL  HISTORY 51 

CHAPTER  VI. 

ICE-BECORDS    7 G3 

CHAPTER  VII. 
MEMORIALS  OF  THE  SEA 72 

CHAPTER  VIII. 
RECORDS  OF  PLANTS  AND  ANIMALS 83 

CHAPTER  IX. 
VOLCANOES  AND  EARTHQUAKES 96 

PAET   II. 
ROCKS,  AND  HOW  THEY  TELL  THE  HISTORY  OF  THE  EARTH. 

CHAPTER  X. 
ELEMENTS  AND  MINERALS 119 

CHAPTER  XI. 
THE  MOBE  IMPOBTANT  ROCKS 145 


4083* 


iv  CONTENTS 

PART  III. 
THE  STRUCTURE  OF  THE  CRUST  OF  THE  EARTH. 

CHAPTER  XII. 

Page 

SEDIMENTARY  ROCKS  179 

CHAPTER  XIII. 
SEDIMENTARY  ROCKS  AFTER  FORMATION 193 

CHAPTER  XIV. 
ERUPTIVE  ROCKS  AND  MINERAL  VEINS 208 

CHAPTER  XV. 
FOSSIL  REMAINS   220 

PART  IV. 

THE  GEOLOGICAL  RECORD  OF  THE  HISTORY  OF  THE 
EARTH. 

CHAPTER  XVI. 
EARLIEST  CONDITIONS  OF  THE  GLOBE 234 

CHAPTER  XVII. 
PALAEOZOIC    PERIODS  —  CAMBRIAN 246 

CHAPTER  XVIII. 
THE  SILURIAN  PERIOD 258 

CHAPTER  XIX. 
DEVONIAN  AND  OLD  RED  SANDSTONE 268 

CHAPTER  XX. 
THE  CARBONIFEROUS  PERIOD 277 

CHAPTER  XXI. 
THE  PERMIAN  ROCKS 295 

CHAPTER  XXII. 
MESOZOIO  PERIODS  —  TRIASSIC 302 

CHAPTER  XXIII. 
THE  JURASSIC  PERIOD.  .  .  310 


CONTENTS  V 

CHAPTER  XXIV. 

Page 
THE  CRETACEOUS  PERIOD 324 

CHAPTER  XXV. 
THE  TERTIARY  PERIODS 338 

CHAPTER  XXVI. 
TERTIARY    PERIODS  —  CONTINUED 350 

CHAPTER  XXVII. 

POST-TERTIARY  PERIODS 363 

INDEX  .  383 


GEOLOGY. 


CHAPTEE  I. 

INTRODUCTORY. 

THE  main  features  of  the  dry  land  on  which  we  live  seem 
to  remain  unchanged  from  year  to  year.  The  valleys 
and  plains  familiar  to  our  forefathers  are  still  familiar 
to  us,  bearing  the  same  meadows  and  woodlands,  the  same 
hamlets  and  villages,  though  generation  after  generation  of 
men  has  meanwhile  passed  away.  The  hills  and  mountains 
now  rise  along  the  sky-line  as  they  did  long  centuries  ago, 
catching  as  of  old  the  fresh  rains  of  heaven  and  gathering 
them  into  the  brooks  and  rivers  which,  through  unknown  ages, 
have  never  ceased  to  flow  seawards.  So  steadfast  do  these 
features  appear  to  stand,  and  so  strong  a  contrast  do  they 
offer  to  the  shortness  and  changeableness  of  human  life,  that 
they  have  become  typical  in  our  minds  of  all  that  -is  ancient 
and  durable.  We  speak  of  the  firm  earth,  of  the  everlasting 
hills,  of  the  imperishable  mountains,  as  if,  where  all  else  is 
fleeting  and  mutable,  these  forms  at  least  remain  unchanged. 

And  yet  attentive  observation  of  what  takes  place  from  day 
to  day  around  us  shows  that  the  surface  of  a  country  is  not 
now  exactly  as  it  used  to  be.  We  notice  various  changes  of 
its  topography  going  on  now,  which  have  doubtless  been  in 
progress  for  a  long  time,  and  the  accumulated  effect  of  which 
may  ultimately  transform  altogether  the  character  of  a  land- 
scape. A  strong  gale,  for  instance,  will  level  thousands  of 
trees  in  its  pathway,  turning  a  tract  of  forest  or  woodland  into 
a  bare  space,  which  may  become  a  quaking  morass,  until  per- 
haps changed  into  arable  ground  by  the  farmer.  A  flooded 
river  will  in  a  few  hours  cut  away  large  slices  from  its  banks, 
and  spreading  over  fields  and  meadows,  will  bury  many  acres 


2  GEOLOGY 

of  fertile  land  under  a  covering  of  barren  sand  and  shingle. 
A  long-continued,  heavy  rain,  by  loosening  masses  of  earth  or 
rock  on  steep  slopes,  causes  destructive  landslips.  A  hard  frost 
splinters  the  naked  fronts  of  crags  and  cliffs,  and  breaks  up 
bare  soil.  In  short,  every  shower  of  rain  and  gust  of  wind, 
if  we  could  only  watch  them  narrowly  enough,  would  be  found 
to  have  done  something  towards  modifying  the  surface  of  the 
land.  Along  the  sea-margin,  too,  how  ceaseless  is  the  progress 
of  change!  In  most  places,  the  waves  are  cutting  away  the 
land,  sometimes  even  at  so  fast  a  rate  as  two  or  three  feet 
in  a  year.  Here  and  there,  on  the  other  hand,  they  cast  sand 
and  silt  ashore  so  as  to  increase  the  breadth  of  the  dry  land. 

These  are  ordinary  everyday  causes  of  alteration,  and  though 
singly  insignificant  enough,  their  united  effect  after  long  cen- 
turies cannot  but  be  great.  Prom  time  to  time,  however,  other 
less  frequent  but  more  powerful  influences  come  into  play.  In 
most  large  regions  of  the  globe,  the  ground  is  often  convulsed 
by  earthquakes,  many  of  which  leave  permanent  scars  upon 
the  surface  of  the  land.  Volcanoes,  too,  in  many  countries 
pour  forth  streams  of  molten  rock  and  showers  of  dust  and 
cinders  that  bury  the  surrounding  districts  and  greatly  alter 
their  appearance. 

Turning  to  the  pages  of  human  history,  we  find  there  the 
records  of  similar  changes  in  bygone  times.  Lakes,  on  which 
our  rude,  forefathers  paddled  their  canoes  and  built  their 
wattled  island-dwellings,  have  wholly  disappeared.  Bogs,  over 
whose  treacherous  surface  these  early  hunters  could  not  follow 
the  chase  of  red  deer  or  Irish  elk,  have  become  meadows  and 
fields.  Forests,  where  they  hunted  the  wild  boar,  have  been 
turned  into  grassy  pastures.  Cities  have  been  entirely  destroyed 
by  earthquakes  or  have  been  entombed  under  the  piles  of  ashes 
discharged  from  a  burning  mountain.  So  great  have  been  the 
inroads  of  the  sea  that,  in  some  instances,  the  sites  of  what  a 
few  hundred  years  ago  were  farms  and  hamlets,  now  lie  under 
the  sea  half  a  mile  or  more  from  the  modern  shore.  .  Else- 
where the  land  has  gained  upon  the  sea,  and  the  harbours 
of  an  earlier  time  are  now  several  miles  distant  from  the 
coast-line. 

But  man  has  naturally  kept  note  only  of  the  more  impressive 
changes;  in  other  words,  of  those  which  had  most  influence 


INTRODUCTORY  3 

upon  his  own  doings.  We  may  be  certain,  however,  that  there 
have  been  innumerable  minor  alterations  of  the  surface  of  the 
land  within  human  history,  of  which  no  chronicler  has  made 
mention,  either  because  they  seemed  too  trivial,  or  because 
they  took  place  so  imperceptibly  as  never  to  be  noticed.  For- 
tunately, in  many  cases,  these  mutations  of  the  land  have 
written  their  own  memorials,  which  can  be  as  satisfactorily  in- 
terpreted as  the  ancient  manuscripts  from  which  our  early  na- 
tional history  is  compiled. 

In  illustration  of  the  character  of  these  natural  chronicles, 
let  us  for  a  moment  consider  the  subsoil  beneath  cities  that 
have  been  inhabited  for  many  centuries.  In  London,  for  ex- 
ample, when  excavations  are  made  for  drainage,  building,  or 
other  purposes,  there  are  sometimes  found,  many  feet  below 
the  level  of  the  present  streets,  mosaic  pavements  and  founda- 
tions, together  with  earthen  vessels,  bronze  implements,  orna- 
ments, coins,  and  other  relics  of  Eoman  time.  Now,  if  we 
knew  nothing,  from  actual  authentic  history,  of  the  existence 
of  such  a  people  as  the  Eomans,  or  of  their  former  presence 
in  England,  these  discoveries,  deep  beneath  the  surface  of  mod- 
ern London,  would  prove  that  long  before  the  present  streets 
were  built,  the  site  of  the  city  was  occupied  by  a  civilised  race 
which  employed  bronze  and  iron  for  the  useful  purposes  of  life, 
had  a  metal  coinage,  and  showed  not  a  little  artistic  skill  in 
its  pottery,  glass,  and  sculpture.  But  down  beneath  the  rubbish 
wherein  the  Eoman  remains  are  embedded,  lie  gravels  and 
sands  from  which  rudely-fashioned  human  implements  of  flint 
have  been  obtained.  Whence  we  further  learn  that,  before  the 
civilised  metal-using  people  appeared,  an  earlier  race  had  been 
there,  which  employed  weapons  and  instruments  of  roughly 
chipped  flint. 

That  this  was  the  order  of  appearance  of  the  successive 
peoples  that  have  inhabited  the  site  of  London  is,  of  course, 
obvious.  But  let  us  ask  ourselves  why  it  is  obvious.  We  observe 
that  there  are,  broadly  speaking,  three  layers  or  deposits  from 
which  the  evidence  is  derived.  The  upper  layer  is  that  which 
contains  the  foundations  and  rubbish  of  modern  London.  Next 
comes  that  which  encloses  the  relics  of  the  Eoman  occupation. 
At  the  bottom  lies  the  layer  that  preserves  the  scanty  traces  of 
the  early  flint-folk.  The  upper  deposit  is  necessarily  the  newest,. 


4  GEOLOGY 

for  it  could  not  be  laid  down  until  after  the  accumulation  of 
those  below  it,  which  must,  of  course,  be  progressively  older, 
as  they  are  traced  deeper  from  the  surface.  By  the  mere  fact 
that  the  layers  lie  one  above  another,  we  are  furnished  with  a 
simple  clue  which  enables  us  to  determine  their  relative  time 
of  formation.  We  may  know  nothing  whatever  as  to  how  old 
they  are  measured  by  years  or  centuries.  But  we  can  be  abso- 
lutely certain  of  what  is  termed  their  "  order  of  superposition/' 
or  chronological  sequence;  in  other  words,  we  can  be  confident 
that  the  bottom  layer  came  first  and  the  top  layer  last. 

This  kind  of  observation  and  reasoning  will  enable  us  to 
detect  almost  everywhere  proofs  that  the  surface  of  the  land 
has  not  always  been  what  it  is  to-day.  In  some  districts,  for 
example,  when  the  dark  layer  of  vegetable  soil  is  turned  up 
which  supports  the  plants  that  keep  the  land  so  green,  there 
may  be  found  below  it  sand  and  gravel,  full  of  smooth  well- 
rounded  stones.  Such  materials  are  to  be  seen  in  the  course 
of  formation  where  water  keeps  them  moving  to  and  fro,  as 
on  the  beds  of  rivers,  the  margins  of  lakes,  or  the  shores  of 
the  sea.  Wherever  smoothed  rolled  pebbles  occur,  they  point 
to  the  influence  of  moving  water;  so  that  we  conclude,  even 
though  the  site  is  now  dry  land,  that  the  sand  and  gravel 
underneath  it  prove  it  to  have  been  formerly  under  water. 
Again,  below  the  soil  in  other  regions,  lie  layers  of  oysters  and 
other  sea-shells.  These  remains,  spread  out  like  similar  shells 
on  the  beach  or  bed  of  the  sea  at  the  present  day,  enable  us  to 
infer  that  where  they  lie  the  sea  once  rolled. 

Pits,  quarries,  or  other  excavations  that  lay  open  still  deeper 
layers  of  material,  bring  before  us  interesting  and  impressive 
testimony  regarding  the  ancient  mutations  of  the  land.  Suppose, 
by  way  of  further  illustration,  that  underneath  a  bed  of  sand 
full  of  oyster-shells,  there  lies  a  dark  brown  band  of  peat.  This 
substance,  composed  of  mosses  and  other  water-loving  plants, 
is  formed  in  boggy  places  by  the  growth  of  marshy  vegetation. 
Below  the  peat  there  might  occur  a  layer  of  soft  white  marl 
full  of  lake-shells,  such  as  may  be  observed  on  the  bottoms  of 
many  lakes  at  the  present  time  (compare  Fig.  30).  These  three 
layers  —  oyster-bed,  peat,  and  marl  —  would  present  a  perfectly 
clear  and  intelligible  record  of  a  curious  series  of  changes  in 
the  site  of  the  locality.  The  bottom  layer  of  white  marl  with 


INTRODUCTORY  5 

its  peculiar  shells  would  show  that  at  one  time  the  place  was 
occupied  by  a  lake.  The  next  layer  of  peat  would  indicate  that, 
by  the  growth  of  marshy  vegetation,  the  lake  was  gradually 
changed  into  a  morass.  The  upper  layer  of  oyster-shells  would 
prove  that  the  ground  was  then  submerged  beneath  the  sea.  The 
present  condition  of  the  ground  shows  that  subsequently  the 
sea  retired  and  the  locality  passed  into  dry  land  as  it  is  to-day. 

It  is  evident  that  by  this  method  of  examination  information 
may  be  gathered  regarding  early  conditions  of  the  earth's  sur- 
face, long  before  the  authentic  dates  of  human  history.  Such 
inquiries  form  the  subject  of  Geology,  which  is  the  science  that 
investigates  the  History  of  the  Earth.  The  records  in  which 
this  history  is  chronicled  are  the  soils  and  rocks  under  our  feet. 
It  is  the  task  of  the  geologist  so  to  arrange  and  interpret  these 
records  as  to  show  through  what  successive  changes  the  globe 
has  passed,  and  how  the  dry  land  has  come  to  wear  the  aspect 
which  it  presents  at  the  present  time. 

Just  as  the  historian  would  be  wholly  unable  to  decipher  the 
inscriptions  of  an  ancient  race  of  people  unless  he  had  first 
discovered  a  key  to  the  language  in  which  they  are  written,  so 
the  geologist  would  find  himself  baffled  in  his  efforts  to  trace 
backward  the  history  of  the  earth  if  he  were  not  provided  with 
a  clue  to  the  interpretation  of  the  records  in  which  that  history 
is  contained.  Such  a  clue  is  furnished  to  him  by  a  study  of 
the  operations  of  nature  now  in  progress  upon  the  earth's  surface. 
Only  in  so  far  as  he  makes  himself  acquainted  with  these  modern 
changes,  can  he  hope  to  follow  intelligently  and  successfully 
the  story  of  earlier  phases  in  the  earth's  progress.  It  will  be 
seen  that  this  truth  has  already  been  illustrated  in  the  instances 
above  given  of  the  evidence  that  the  surface  of  the  land  has 
not  been  always  as  it  is  now.  The  beds  of  sand  and  gravel,  of 
oyster-shells,  of  peat  and  of  marl,  would  have  told  us  nothing 
as  to  ancient  geography  had  we  not  been  able  to  ascertain  their 
origin  and  history  by  finding  corresponding  materials  now  in 
course  of  accumulation.  To  one  ignorant  of  the  peculiarities 
of  fresh-water  shells,  the  layer  of  marl  would  have  conveyed 
no  intelligible  meaning.  But  knowing  and  recognising  these 
peculiarities,  we  feel  sure  that  the  marl  marks  the  site  of  a 
former  lake.  Thus  the  study  of  the  Present  supplies  a  key  that 
unlocks  the  secrets  of  the  Past. 


6  GEOLOGY 

In  order,  therefore,  to  trace  back  the  history  of  the  Earth, 
the  geologist  must  begin  by  carefully  watching  the  changes 
that  now  take  place,  and  by  observing  how  nature  elaborates  the 
materials  that  preserve  more  or  less  completely  the  record  of 
these  changes.  In  the  following  pages,  I  propose  to  follow 
this  method  of  inquiry,  and,  as  far  as  the  subject  will  permit,  to 
start  with  no  assumptions  which  the  learner  cannot  easily  verify 
for  himself.  We  shall  begin  with  the  familiar  everyday  opera- 
tions of  the  air,  rain,  frost,  and  other  natural  agents.  It  will  not 
be  needful  here  to  consider  them  in  detail.  We  shall  rather 
pass  on  to  inquire  in  what  various  ways  they  are  engaged  in 
contributing  to  the  formation  of  new  mineral  accumulations, 
and  in  thereby  providing  fresh  materials  for  the  preservation 
of  the  facts  on  which  geological  history  is  founded.  Having 
thus  traced  how  new  rocks  are  formed,  we  may  then  proceed 
to  arrange  the  similar  rocks  of  older  time,  marking  what  are 
the  peculiarities  of  each  and  how  they  may  best  be  classified. 

If  the  labours  of  the  geologist  were  concerned  merely  with 
the  former  mutations  of  the  earth's  surface, —  how  sea  and  land 
have  changed  places,  how  rivers  have  altered  their  courses,  how 
lakes  have  been  filled  up,  how  valleys  have  been  excavated,  how 
mountains,  peaks,  and  precipices  have  been  carved,  how  plains 
have  been  spread  out,  and  how  the  story  of  these  revolutions 
has  been  written  in  enduring  characters  upon  the  very  frame- 
work of  the  land, —  he  would  feel  the  want  of  one  of  the  great 
sources  of  interest  in  the  study  of  the  present  face  of  nature. 
We  naturally  connect  all  modern  changes  of  the  earth's  surface 
with  the  life  of  the  plants  and  animals  that  flourish  there,  and 
more  especially  with  their  influence  on  the  progress  of  Man 
himself.  If  there  were  no  similar  connection  of  the  ancient 
changes  with  once  living  things  —  if  the  history  of  the  earth 
were  merely  one  of  dead  inert  matter  it  would  lose  much  of 
its  interest  for  us.  But  happily  that  history  includes  the  records 
of  successive  generations  of  plants  and  animals  which,  from 
early  times,  have  peopled  land  and  sea.  The  remains  of  these 
organisms  have  been  preserved  in  the  deposits  of  different  ages, 
and  can  be  compared  and  contrasted  with  those  of  the  modern 
world. 

To  realise  how  such  preservation  has  been  possible,  and  how 
far  the  forms  so  retained  afford  an  adequate  picture  of  the  life 


INTRODUCTORY  7 

of  the  time  to  which  they  belonged,  we  must  turn  once  more 
to  watch  how  nature  deals  with  this  matter  at  the  present  time. 
Of  the  millions  of  flowers,  shrubs,  and  trees  which  year  after 
year  clothe  the  land  with  beauty,  how  many  relics  are  preserved  ? 
Where  are  the  successive  generations  of  insect,  bird,  and  beast 
which  have  appeared  in  this  country  since  man  first  set  foot 
upon  its  soil?  They  have  utterly  vanished.  If  all  their  living 
descendants  could  suddenly  be  swept  away,  how  could  we  tell 
that  such  plants  and  animals  ever  lived  at  all?  It  must  be 
confessed  that  the  vast  majority  of  them  leave  no  trace  behind. 
Nevertheless  we  should  be  able  to  recover  relics  of  some  of 
them  by  searching  in  the  comparatively  few  places  where,  at 
the  present  day,  dead  plants  and  animals  are  entombed  and 
preserved.  From  the  alluvial  terraces  of  rivers,  from  the  silt  of 
lake-bottoms,  from  the  depths  of  peat-mosses,  from  the  floors 
of  subterranean  caverns,  from  the  incrustations  left  by  springs, 
we  might  recover  traces  of  some  at  least  of  the  living  things 
that  people  the  land.  And  from  these  fragmentary  and  incom- 
plete records  we  might  conjecture  what  may  have  been  the  gen- 
eral character  of  the  life  of  the  time.  By  searching  the  similar 
records  of  earlier  ages  the  geologist  has  brought  to  light  many 
profoundly  interesting  vestiges  of  vegetation  and  of  animal  life 
belonging  to  types  that  have  long  since  passed  away. 

It  must  be  evident,  however,  that  were  we  to  confine  our 
inquiries  merely  to  its  surface,  we  should  necessarily  gain  a 
most  imperfect  view  of  the  general  history  of  the  Earth.  Be- 
neath that  surface,  as  volcanoes  show,  there  lies  a  hot  interior, 
which  must  have  profoundly  influenced  the  changes  of  the  outer 
parts  or  crust  of  the  planet.  The  study  of  volcanoes  enables  us 
to  penetrate,  as  it  were,  a  little  way  into  that  interior,  and  to 
understand  some  of  the  processes  in  progress  there.  But  our 
knowledge  of  the  inside  of  the  Earth  can  obviously  be  based 
only  to  a  very  limited  extent  on  direct  observation,  for  man 
cannot  penetrate  far  below  the  surface.  The  deepest  mines  do 
not  go  deep  enough  to  reach  materials  differing  in  any  essential 
respect  from  those  visible  above  ground.  Nevertheless,  by  infer- 
ence from  such  observations  as  can  be  made,  and  by  repeated 
and  varied  experiments  in  laboratories,  imitating  as  closely  as 
can  be  devised  what  may  be  supposed  to  be  the  conditions  that 
exist  deep  within  the  globe,  some  probable  conclusions  can  be 


8  GEOLOGY 

drawn  even  as  to  the  changes  that  take  place  in  those  deeper 
recesses  that  lie  for  ever  concealed  from  our  eyes.  These  con- 
clusions will  be  stated  in  later  chapters  of  this  book,  and  the 
rocks  will  be  described,  on  the  origin  of  which  they  appear  to 
throw  light. 

I  have  compared  the  soils  and  rocks  with  which  geology  deals 
to  the  records  out  of  which  the  historian  writes  the  chronicles 
of  a  nation.  We  might  vary  the  simile  by  likening  them  to  the 
materials  employed  in  the  construction  of  a  great  building.  It 
is  of  course  interesting  enough  to  know  what  kinds  of  marble, 
granite,  mortar,  wood,  brass,  or  iron,  have  been  chosen  by  an 
architect.  But  much  more  important  is  it  to  inquire  how  these 
various  substances  have  been  grouped  together  so  as  to  form 
such  a  building.  In  like  manner,  besides  the  nature  and  mode 
of  origin  of  the  various  rocks  of  which  the  visible  and  accessible 
part  of  the  earth  consists,  we  ought  to  know  how  these  varied 
substances  have  been  arranged  so  as  to  build  up  what  we  can 
see  of  the  outer  part  or  crust  of  our  globe.  In  short,  we  should 
try  to  trace  what  may  be  called  the  architecture  of  the  planet, 
noting  how  each  variety  of  rock  occupies  its  own  characteristic 
place,  and  how  they  are  all  grouped  and  braced  together  in  the 
solid  framework  of  the  land.  This  then  will  be  the  next  subject 
for  consideration  in  this  volume. 

'  But  in  a  great  historical  edifice,  like  one  of  the  Gothic  min- 
sters of  Europe,  for  example,  there  are  often  several  different 
styles.  A  student  of  architecture  can  detect  these  distinctions, 
and  by  their  means  can  show  that  a  cathedral  has  not  been 
completed  in  one  age;  that  it  may  even  have  been  partially 
destroyed  and  rebuilt  during  successive  centuries,  only  finally 
taking  its  present  form  after  many  political  vicissitudes  and 
many  changes  of  architectural  taste.  Each  edifice  has  thus  a 
separate  history,  which  is  recorded  by  the  way  the  materials 
have  been  shaped  and  put  together  in  the  various  parts  of  the 
masonry.  So  it  is  with  the  architecture  of  the  Earth.  We 
have  evidence  of  many  demolitions  and  rebuildings,  and  the 
story  of  their  general  progress  can  still  be  deciphered  among 
the  rocks.  It  is  the  business  of  Geology  to  trace  out  that  story, 
to  put  all  the  scattered  materials  together,  and  to  make  known 
by  what  a  long  succession  of  changes  the  Earth  has  reached 
its  present  state.  An  outline  of  what  science  has  accomplished 


INTRODUCTORY  9 

in  this  task  will  form  the  last  and  concluding  part  of  this  book. 
In  the  following  chapters  I  wish  two  principles  to  be  kept 
steadily  in  View.  In  the  first  place,  looking  upon  Geology  as 
the  study  of  the  Earth's  history,  we  need  not  at  first  concern 
ourselves  with  any  details,  save  those  that  may  be  needed  to 
enable  us  clearly  to  understand  what  the  general  character  and 
progress  of  this  history  have  been.  In  a  science  which  em- 
braces so  vast  a  range  as  Geology,  the  multiplicity  of  facts  to 
be  examined  and  remembered  may  seem  at  first  to  be  almost 
overwhelming.  But  a  selection  of  the  essential  facts  is  sufficient 
to  give  the  learner  a  clear  view  of  the  general  principles  and 
conclusions  of  the  science,  and  to  enable  him  to  enter  with 
intelligence  and  interest  into  more  detailed  treatises.  In  the 
second  place,  Geology  is  essentially  a  science  of  observation. 
The  facts  with  which  it  deals  should,  as  far  as  possible,  be 
verified  by  our  own  personal  examination.  We  should  lose  no 
opportunity  of  seeing  with  our  own  eyes  the  actual  progress  of 
the  changes  which  it  investigates,  and  the  proofs  which  it  adduces 
of  similar  changes  in  the  far  past.  To  do  this  will  lead  us 
into  the  fields  and  hills,  to  the  banks  of  rivers  and  lakes,  and 
to  the  shores  of  the  sea.  We  can  hardly  take  any  country  walk, 
indeed,  in  which  with  duly  observant  eye  we  may  not  detect 
either  some  geological  operation  in  actual  progress,  or  the  evi- 
dence of  one  which  was  completed  long  ago.  Having  learnt 
what  to  look  for  and  how  to  interpret  it  when  seen,  we  are 
as  it  were  gifted  with  a  new  sense.  Every  landscape  comes  to 
possess  a  fresh  interest  and  charm,  for  we  carry  about  with  us 
everywhere  an  added  power  of  enjoyment,  whether  the  scenery 
has  long  been  familiar  or  presents  itself  for  the  first  time.  I 
would  therefore  seek  at  the  outset  to  impress  upon  those  who 
propose  to  read  the  following  pages,  that  one  of  the  main 
objects  with  which  this  book  is  written  is  to  foster  a  habit  of 
observation,  and  to  serve  as  a  guide  to  what  they  are  them- 
selves to  look  for,  rather  than  merely  to  relate  what  has  been 
seen  and  determined  by  others.  If  they  will  so  learn  these 
lessons,  I  feel  sure  that  they  will  never  regret  the  time  and 
labour  they  may  spend  over  the  task. 


10  GEOLOGY 


PART  I. 

THE  MATEKIALS  FOR  THE  HISTOEY  OF  THE  EAETH. 
CHAPTER  II. 

INFLUENCE  OF  THE  ATMOSPHERE   IN  THE  CHANGES  OF   THE 
EARTH'S  SURFACE. 

IN  the  history  of  mankind  no  sharp  line  can  be  drawn  between 
the  events  that  are  happening  now  or  have  happened  within 
the  last  few  generations,  and  those  that  took  place  long 
ago,  and  which  are  sometimes,  though  inaccurately,  spoken  of 
as  historical.  Every  people  is  enacting  its  history  today  just 
as  fully  as  it  did  many  centuries  ago.  The  historian  recognises 
this  continuity  in  human  progress.  He  knows  that  the  feelings 
and  aspirations  which  guided  mankind  in  old  times  were  essen- 
tially the  same  influences  that  impel  them  now,  and  therefore 
that  the  wider  his  knowledge  of  his  fellowmen  of  the  present 
day,  the  broader  will  be  his  grasp  in  dealing  with  the  transac- 
tions of  former  generations.  So  too  is  it  with  the  history  of 
the  Earth.  That  history  is  in  progress  now  as  really  as  it  has 
ever  been,  and  its  events  are  being  recorded  in  the  same  way 
and  by  the  same  agents  as  in  the  far  past.  Its  continuity  has 
never  been  broken.  Obviously,  therefore,  if  we  would  explore 
its  records  fe  in  the  dark  backward  and  abysm  of  time,"  we 
should  first  make  ourselves  familiar  with  the  manner  in  which 
these  records  are  being  written  from  day  to  day  before  our 
eyes. 

In  this  first  Part,  attention  will  accordingly  be  given  to  the 
changes  in  progress  upon  the  Earth  at  the  present  time,  and 
to  the  various  ways  in  which  the  passing  of  these  changes  is 
chronicled  in  natural  records.  We  shall  watch  the  actual  trans- 
action of  geological  histor}^,  and  mark  in  what  way  its  incidents 


INFLUENCE  OF  THE  ATMOSPHERE  11 

inscribe  themselves  on  the  page  of  the  earth's  surface.  Every- 
day and  hour  witness  the  enacting  of  some  geological  event, 
trifiing  and  transient  or  stupendous  and  durable.  Sometimes 
the  event  leaves  behind  it  only  an  imperceptible  trace  of  its 
passage,  at  other  times  it  graves  itself  almost  imperishably  in 
the  annals  of  tne  globe.  In  tracing  the  origin  and  development 
of  these  geological  annals  of  the  present  time,  we  shall  best 
qualify  ourselves  for  deciphering  the  records  of  the  early  revo- 
lutions of  the  planet.  We  are  thereby  led  to  study  the  various 
chronicles  compiled  respectively  by  the  air,  rain,  rivers,  springs, 
glaciers,  the  sea,  plants  and  animals,  volcanoes  and  earthquakes 
—  in  other  words,  all  the  deposits  left  by  the  operations  of 
these  agents,  the  scars  or  other  features  made  by  them  upon 
the  earth's  surface,  and  all  other  memorials  of  geological  change. 
Having  learnt  how  modern  deposits  are  produced,  and  how  they 
preserve  the  story  of  their  origin,  we  shall  then  be  able  to  group 
with  them  the  corresponding  deposits  of  earlier  times,  and  to 
embrace  all  the  geological  records,  ancient  as  well  as  modern, 
in  one  general  scheme  of  classification.  Such  a  scheme  will 
enable  us  to  see  the  continuity  of  the  materials  of  geological 
history,  and  will  fix  definitely  for  us  the  character  and  relative 
position  of  all  the  chief  rocks  out  of  which  the  visible  part  of 
the  globe  is  composed. 

WEATHERING. —  The  gradual  change  that  overtakes  everything 
on  the  face  of  the  earth  is  expressed  in  all  languages  by  familiar 
phrases  which  imply  that  the  mere  passing  of  time  is  the  cause 
of  the  change.  As  Sir  Thomas  Browne  quaintly  said  more  than 
two  hundred  years  ago,  "time  antiquates  antiquities,  and  hath 
an  art  to  make  dust  of  all  things."  We  speak  of  the  dust  of 
antiquity  and  the  gnawing  tooth  of  time.  We  say  that  things 
are  time-eaten,  worn  with  age,  crumbling  under  a  weight  of 
years.  Nothing  suggests  such  epithets  so  strikingly  as  an  old 
building.  We  know  that  the  masonry  at  first  was  smooth  and 
fresh;  but  now  we  describe  it  as  weather-beaten,  decayed,  cor- 
roded. So  distinctive  is  this  appearance  that  it  is  always  looked 
for  in  an  ancient  piece  of  stone-work;  and  if  not  seen,  its 
absence  at  once  suggests  a  doubt  whether  the  masonry  can  really 
be  old.  No  matter  of  what  varieties  of  stone  the  edifice  may 
have  been  built,  a  few  generations  may  be  enough  to  give  them 
this  look  of  venerable  antiquity.  The  surface  that  was  left 


12  GEOLOGY 

smoothly  polished  by  the  builders  grows  rough  and  uneven,  with 
scars  and  holes  eaten  into  it.  Portions  of  the  original  polish 
that  may  here  and  there  have  escaped,  serve  as  a  measure  of 
how  much  has  actually  been  removed  from  the  rest  of  the 
surface. 

Now,  if  in  the  lapse  of  time,  stone  which  has  been  artificially 
dressed  is  wasted  away,  we  may  be  quite  certain  that  the  same 
stone  in  its  natural  position  on  the  slope  of  a  hill  or  valley, 
or  by  the  edge  of  a  river  or  of  the  sea,  must  decay  in  a  similar 

way.  Indeed,  an  examination  of 
any  crumbling  building  will  show 
that,  in  proportion  as  the  chiselled 
surface  disappears,  the  stone  puts 
on  the  ordinary  look  which  it  wears 
where  it  has  never  been  cut  by  man, 
and  where  only  the  finger  of  time 
has  touched  it.  Could  we  remove 
some  of  the  decayed  stones  from 
the  building  and  insert  them  into  a 
natural  crag  or  cliff  of  the  same 

Fig.  1.  —  Weathering  of  rock,  as    u-^j  of  cfrmp    fhpir  Twnliar  tirnp- 

Kin  Q6    l 


shown  by  old  masonry.    (The 

"  false-bedding  "  and  other  worn  aspect  would  be  found  to  be 
STare  "reveal  'by  "weatt  so  exactly  that  of  the  rest  of  the  cliff 
ering.)  that  probably  no  one  would  ever 

suspect  that  a  mason's  tools  had  once  been  upon  them. 

From  this  identity  of  surface  between  the  time-worn  stones 
of  an  old  building  and  the  stone  of  a  cliff  we  may  confidently 
infer  that  the  decay  so  characteristic  of  ancient  masonry  is  as 
marked  upon  natural  faces  of  rock.  The  gradual  disappearance 
of  the  artificial  smoothness  given  by  the  mason,  and  its  replace- 
ment by  the  ordinary  natural  rough  surface  of  the  stone,  shows 
that  this  natural  surface  must  also  be  the  result  of  decay.  And 
as  the  peculiar  crumbling  character  is  universal,  we  may  be 
sure  that  the  decay  with  which  it  is  connected  must  be  general 
over  the  globe. 

But  the  mere  passing  of  time  obviously  cannot  change  any- 
thing, and  to  say  that  it  does  is  only  a  convenient  figure  of 
speech.  It  is  not  time^  but  the  natural  processes  which  require 
time  for  their  work,  that  produce  the  widespread  decay  over 
the  surface  of  the  earth.  Of  these  natural  processes,  there  are 


INFLUENCE  OF  THE  ATMOSPHERE  13 

four  that  specially  deserve  consideration  —  changes  of  tempera- 
ture,, saturation  and  desiccation,  frost,  and  rain. 

(1)  Changes    of    Temperature. —  In    countries    where    the 
days  are  excessively  hot,  with  nights  correspondingly  cool,  the 
surfaces  of  rocks  heated  sometimes,  as  in  parts  of  Africa,  up  to 
more  than  130°  Fahr.  by  a  tropical  sun,  undergo  considerable 
expansion  in  consequence  of  this  increase  of  temperature.     At 
night,  on  the  other  hand,  the  rapid  radiation  quickly  chills  the 
stone  and  causes  it  to  contract.     Hence  the  superficial  parts, 
being  in  a  perpetual  state  of  strain,  gradually  crack  up  or  peel 
off.    The  face  of  a  cliff  is  thus  worn  slowly  backward,  and  the 
prostrate  blocks  that  fall  from  it  are  reduced  to  smaller  frag- 
ments and  finally  to  dust.    Where,  as  in  Europe  and  the  settled 
parts  of  North  America,  the  contrasts  of  temperature  are  not 
so  marked,  the  same  kind  of  waste  takes  place  in  a  less  striking 
manner. 

(2)  Saturation  and  Desiccation. —  Another  cause  of  the  decay 
of  the  exposed  surfaces  of  rocks  is  to  be  sought  in  the  alter- 
nate soaking  of  them  with  rain  and  drying  of  them  in  sunshine, 
whereby  the  component  particles  of  the  stone  are  loosened  and 
fall  to  powder.     Some  kinds  of  stone  freshly  quarried  and  left 
to  this  kind  of  action  are  rapidly  disintegrated.     The  rock 
called  shale  is  peculiarly  liable  to  decay  from  this  cause.     The 
cliffs  into  which  it  sometimes  rises  show  at  their  base  long  trails 
of  rubbish  entirely  derived  from  its  waste. 

(3)  Frost. —  A  third  and  familiar  source  of  decay  in  stone 
exposed  to  the  atmosphere  is  to  be  found  in  the  action  of  Frost. 
The  water  that  falls  from  the  air  upon  the  surface  of  the  land 
soaks  into  the  soil  and  into  the  pores  of  rocks.     When  the 
temperature  of  the  air  falls  belong  the  freezing  point,  the  im- 
prisoned moisture  expands  as  it  passes  into  ice,  and  in  expanding 
pushes  aside  the  particles  between  which  it  is  entangled.    Where 
this  takes  place  in  soil,  the  pebbles  and  the  grains  of  sand  and 
earth  are  separated  from  each  other  by  the  ice  that  shoots  be- 
tween them.    They  are  all  frozen  into  a  solid  mass  that  rings 
like  stone  under  our  feet;  but,  as  soon  as  a  thaw  sets  in,  the 
ice  that  formed  the  binding  cement  passes  into  water  which 
converts  the  soil  into  soft  earth  or  mud.    This  process,  repeated 
winter  after  winter,  breaks  up  the  materials  of  the  soil,  and 
enables  them  to  be  more  easily  made  use  of  by  plants  and  more 


14  GEOLOGY 

readily  blown  away  by  wind  or  washed  of?  by  rain.  Where 
the  action  of  frost  affects  the  surface  of  a  rock,  the  particles 
separated  from  each  other  are  eventually  blown  or  washed  away, 
or  the  rock  peels  off  in  thin  crusts  or  breaks  up  into  angular 
pieces,  which  are  gradually  disintegrated  and  removed. 

(4)  Rain. —  One  further  cause  of  decay  may  be  sought  in 
the  remarkable  power  possessed  by  Rain  of  chemically  corrod- 
ing stones.  In  falling  through  the  atmosphere,  rain  absorbs 
the  gases  of  the  air,  and  with  their  aid  attacks  surfaces  of 
rock.  With  the  oxygen  thus  acquired,  it  oxidises  those  substances 
which  can  still  take  more  of  this  gas,  causing  them  to  rust 
(See  Metalloids).  As  a  consequence  of  this  alteration,  the  co- 
hesion of  the  particles  is  usually  weakened,  and  the  stone 
crumbles  down.  With  the  carbon-dioxide,  or  carbonic  acid,  it 
dissolves  and  removes  some  of  the  more  soluble  ingredients  in 
the  form  of  carbonates,  thereby  also  usually  loosening  the  com- 
ponent particles  of  the  stone.  In  general,  the  influence  of  rain 
is  to  cause  the  exposed  parts  of  rocks  to  rot  from  the  surface 
inward.  Where  the  ground  is  protected  with  vegetation,  the 
decay  is  no  doubt  retarded;  but  in  the  absence  of  vegetation, 
the  outer  crust  of  the  decayed  layer  is  apt  to  be  washed  off  by 
rain,  or  when  dried  to  powder  may  be  blown  away  and  scattered 
by  wind.  As  fast  as  it  is  removed  from  the  surface,  however, 
it  is  renewed  underneath  by  the  continued  soaking  of  rain  into 
the  stone. 

Effects  of  Weathering. —  Hence  one  of  the  first  lessons  to 
be  learnt  —  when  from  the  common  evidence  around  us  we  seek 
to  know  what  has  been  the  history  of  the  ground  on  which  we 
live  —  is  one  of  ceaseless  decay.  All  over  the  land,  in  all  kinds 
of  climates,  and  from  various  causes,  bare  surfaces  of  soil  and 
rock  yield  to  the  influences  of  the  atmosphere  or  weather.  The 
decay  thus  set  in  motion  is  commonly  called  "  weathering." 
That  it  may  often  be  comparatively  rapid  is  familiarly  and  in- 
structively shown  in  buildings  or  open-air  monuments  of  which 
the  dates  are  precisely  known.  Marble  tombstones  in  the  grave- 
yards of  large  towns,  for  example,  hardly  keep  their  inscriptions 
legible  for  even  so  long  as  a  century.  Before  that  time,  the 
surface  of  the  stone  has  crumbled  away  into  a  kind  of  sand. 
Everywhere  the  weather-eaten  surfaces,  the  crumbling  crust  of 


INFLUENCE  OF  THE  ATMOSPHERE 


15 


decayed  stone,  and  the  scattered  blocks  and  trains  of  rubbish, 
tell  their  tale  of  universal  waste. 

It  is  well  to  take  numerous  opportunities  of  observing  the 
process  of  this  decay  in  different  situations  and  on  various  kinds 
of  materials.  We  can  thus  best  realise  the  important  part  which 
weathering  must  play  in  the  changes  of  the  earth's  surface,  and 
we  prepare  ourselves  for  the  consideration  of  the  next  question 
that  arises,  What  becomes  of  all  the  rotted  material? — a  ques- 
tion to  answer  which  leads  us  into  the  very  foundations  of 
geological  history. 

Openings  from  the  soil  down  into  the  rock  underneath  often 
afford  instructive  lessons  regarding  the  decay  of  the  surface 
of  the  land.  Fig.  2,  for  instance,  is  a 
drawing  of  one  of  these  sections,  in  which 
a  gradual  passage  may  be  traced  from  solid 
sandstone  (a)  underneath  up  into  broken- 
up  sandstone  (&),  and  thence  into  the 
earthy  layer  (c)  that  supports  the  vegeta- 
tion of  the  surface.  Traced  from  below 
upwards,  the  rock  is  found  to  become  more 
and  more  broken  and  crumbling,  with  an 
increasing  number  of  rootlets  that  strike 
freely  through  it  in  all  directions,  until 

it    passes    insensibly    into    the    uppermost  Fig     2 Passage    of 

dark  layer  of  vegetable  soil  or  humus,  sandstone  upwards 
This  dark  layer  owes  its  characteristic 
brown  or  black  colour  to  the  decaying  remains  of  vegetation 
diffused  through  it.  Again,  granite 
in  its  unweathered  state  is  a  hard, 
compact,  crystalline  rock  that  may 
be  quarried  out  in  large  solid  blocks 
(a  in  Fig.  3),  yet  when  traced  up- 
ward to  within  a  few  feet  from  the 
surface  it  may  be  seen  to  have  been 
split  by  innumerable  rents  into 
fragments  which  are  nevertheless 
still  lying  in  their  original  position. 
As  these  fragments  are  attacked  by 
percolating  moisture,  their  surfaces 
decay,  leaving  the  still  unweathered 


16  GEOLOGY 

parts  as  rounded  blocks  (&),  which  might  at  first  be  mistaken  for 
transported  boulders.  They  are,  however,  part?  of  the  rock  bro- 
ken up  in  place,  and  not  fragments  that  have  been  carried  from  a 
distance.  The  little  quartz  veins  that  traverse  the  solid  granite 
can  be  recognised  running  through  the  decayed  and  fresh  parts 
alike.  But,  besides  being  broken  into  pieces,  the  granite  rots 
away  and  loses  its  cohesion.  Some  of  the  smaller  pieces  can 
be  crumbled  down  between  the  fingers,  and  this  decay  increases 
upwards,  until  the  rock  becomes  a  mere  sand  or  sandy  clay  in 
which  a  few  harder  kernels  are  still  left.  Into  this  soft  layer 
roots  may  descend  from  the  surface,  and,  like  the  sandstone, 
the  granite  merges  above  into  the  overlying  soil  (c). 

Soil  and  Subsoil. —  In  such  sections  as  the  foregoing,  three 
distinct  layers  can  be  recognised  which  pass  into  each  other. 
At  the  bottom  lies  the  rock,  either  undecayed  or  at  least  still 
fresh  enough  to  show  its  true  nature.  Next  comes  the  broken- 
up  crumbling  layer  through  which  stray  roots  descend,  and 
which  is  known  as  the  subsoil.  At  the  top  lies  the  dark  band, 
crowded  with  rootlets  and  forming  the  true  soil.  These  three 
layers  obviously  represent  successive  stages  in  the  decay  of  the 
surface  of  the  land.  The  soil  is  the  layer  of  most  complete 
decay.  The  subsoil  is  an  intermediate  band  where  the  progress 
of  decomposition  has  not  advanced  so  far,  while  the  shattered 
rock  underneath  shows  the  earlier  stages  of  disintegration. 
Vegetation  sends  its  roots  and  rootlets  through  the  rotted  rock. 
As  the  plants  die,  they  are  succeeded  by  others,  and  the  rotted 
remains  of  their  successive  generations  gradually  darken  the 
uppermost  decomposed  layer.  Worms,  insects,  and  larger  ani- 
mals that  may  die  on  the  surface,  likewise  add  their  mouldering 
remains  to  this  uppermost  deposit.  And  thus  from  animals 
and  plants  there  is  furnished  to  the  soil  that  organic  matter  on 
which  its  fertility  so  much  depends.  The  very  decay  of  the 
vegetation  helps  to  promote  that  of  the  underlying  rock,  for 
it  supplies  various  organic  acids  ready  to  be  absorbed  by  per- 
colating rain-water,  the  power  of  which  to  decompose  rocks  is 
thereby  increased. 

It  is  obvious,  then,  that  in  answer  to  the  question,  What  be- 
comes of  the  rotted  material  produced  by  weathering?  we  may 
confidently  assert  that,  over  surfaces  of  land  protected  by  a 
cover  of  vegetation,  this  material  in  large  measure  accumulates 


INFLUENCE  OF  THE  ATMOSPHERE  17 

where  it  is  formed.  Such  accumulation  will  naturally  take 
place  chiefly  on  flat  or  gently  inclined  ground.  Where  the  slope 
is  steep,  the  decomposed  layer  will  tend  to  travel  down-hill  by 
mere  gravitation,  and  to  be  further  impelled  downward  by 
descending  rain-water. 

If  there  is  so  intimate  a  connection  between  the  soil  at  the 
surface  and.  the  rock  underneath,  we  can  readily  understand 
that  soils  should  vary  from  one  district  to  another,  according 
to  the  nature  of  the  underlying  rocks.  Clays  will  produce  clayey 
soil,  sandstones,  sandy  soil,  or,  where  these  two  kinds  of  rock 
occur  together,  they  may  give  rise  to  sandy  clay  or  loam.  Hence, 
knowing  what  the  underlying  rock  is,  we  may  usually  infer 
what  must  be  the  character  of  the  overlying  soil,  or,  from  the 
nature  of  the  soil,  we  may  form  an  opinion  respecting  the  quality 
of  the  rock  that  lies  below. 

But  it  will  probably  occur  to  the  thoughtful  observer  that 
when  once  a  covering  of  soil  and  subsoil  has  been  formed  over 
a  level  piece  of  ground,  especially  where  there  is  also  an  over- 
lying carpet  of  verdure,  the  process  of  decay  should  cease  —  the 
very  layer  of  rotted  material  coming  eventually  to  protect  the 
rock  from  further  disintegration.  Undoubtedly,  under  these 
circumstances,  weathering  is  reduced  to  its  feeblest  condition. 
But  that  it  still  continues  will  be  evident  from 'some  considera- 
tions, the  force  of  which  will  be  better  understood  a  few  pages 
further  on.  If  the  process  were  wholly  arrested,  then  in  course 
of  time  plants  growing  on  the  surface  would  extract  from  the 
soil  all  the  nutriment  they  could  get  out  of  it,  and  with  the 
increasing  impoverishment  of  the  soil,  they  would  dwindle  away 
and  finally  die  out,  until  perhaps  only  the  simpler  forms  of 
vegetation  would  grow  on  the  site.  Something  of  this  kind  • 
not  improbably  takes  place  where  forests  decay  and  are  replaced 
by  scrub  and  grass.  But  the  long-continued  vigorous  growth  of 
the  same  kind  of  plants  upon  a  tract  of  land  doubtless  indicates 
that  in  some  way  the  process  of  weathering  is  not  entirely  ar- 
rested, but  that,  as  generation  succeeds  generation,  the  plants 
are  still  able  to  draw  nutriment  from  fresh  portions  of  decom- 
posed rock.  A  cutting  made  through  the  soil  and  subsoil  shows 
that  roots  force  their  way  downward  into  the  rock,  which  splits 
up  and  allows  percolating  water  to  soak  downwards  through  it. 
The  subsoil  thus  gradually  eats  its  way  into  the  solid  rock 


18  GEOLOGY 

below.  Influences  are  at  work  also,  whereby  there  is  an  imper- 
ceptible removal  of  material  from  the  surface  of  the  soil. 
Notable  among  these  influences  are  Rain,  Wind,  and  Earth- 
worms. 

Wherever  soil  is  bare  of  vegetation  it  is  directly  exposed  to 
removal  by  RAIN.  Ground  is  seldom  so  flat  that  rain  may  not 
flow  a  little  way  along  the  surface  before  sinking  underneath. 
In  its  flow,  it  carries  off  the  finer  particles  of  the  soil.  These 
may  travel  each  time  only  a  short  way,  but  as  the  operation  is 
repeated,  they  are  in  the  course  of  years  gradually  moved  down 
to  lower  ground  or  to  some  runnel  or  brook  that  sweeps  them 
away  seaward.  Both  on  gentle  and  on  steep  slopes,  this  trans- 
porting power  of  rain  is  continually  removing  the  upper  layer 
of  bared  soil. 

Where  soil  is  exposed  to  the  sun,  it  is  liable  to  be  dried  into 
mere  dust,  which  is  borne  off  by  WIND.  How  readily  this  may 
happen  is  often  strikingly  seen  after  dry  weather  in  spring-time. 
The  earth  of  ploughed  fields  becomes  loose  and  powdery,  and 
clouds  of  its  finer  particles  are  carried  up  into  the  air  and  trans- 
ported to  other  farms,  as  gusts  of  wind  sweep  across.  "March 
dust,"  which  is  a  proverbial  expression,  may  be  remembered  as 
an  illustration  of  one  way  in  which  the  upper  parts  of  the  soil 
are  removed. 

Even  where  a  grassy  turf  protects  the  general  surface,  bare 
places  may  always  be  found  whence  this  covering  has  been 
removed.  Rabbits,  moles,  and  other  animals  throw  out  soil 
from  their  burrows.  Mice  sometimes  lay  it  bare  by  eating  the 
pasture  down  to  the  roots.  The  common  EARTHWORMS  bring 
up  to  daylight  in  the  course  of  a  year  an  almost  incredible 
quantity  of  it  in  their  castings.  Mr.  Darwin  estimated  that 
this  quantity  is  in  some  places  not  less  than  10  tons  per  annum 
over  an  acre  of  ground.  Only  the  finest  particles  of  mould 
are  swallowed  by  worms  and  conveyed  by  them  to  the  surface, 
and  it  is  precisely  these  which  are  most  apt  to  be  washed  off 
by  rain  or  to  be  dried  and  blown  away  as  dust  by  the  wind. 
Where  it  remains  on  the  ground,  the  soil  brought  up  by  worms 
covers  over  stones  and  other  objects  lying  there,  which  con- 
sequently seem  to  sink  into  the  earth.  The  operation  of  these 
animals  causes  the  materials  of  the  soil  to  be  thoroughly  mixed. 
In  tropical  countries,  the  termite  or  "white  ant"  conveys  a 


INFLUENCE  OF  THE  ATMOSPHERE          19 

prodigious  amount  of  fine  earth  up  into  the  open  air.  With 
this  material  it  builds  hills  sometimes  60  feet  high  and  visible 
for  a  distance  of  several  miles;  likewise  tunnels  and  chambers, 
which  it  plasters  all  over  the  stems  and  branches  of  trees,  often 
so  continuously  that  hardly  any  bark  can  be  seen.  The  fine 
soil  thus  exposed  is  liable  to  be  blown  away  by  the  wind  or 
washed  off  by  the  fierce  tropical  rains. 

Although,  therefore,  the  layer  of  vegetable  soil  which  covers 
the  land  appears  to  be  a  permanent  protection,  it  does  not  really 
prevent  a  large  amount  of  material  from  being  removed  even 
from  grassy  ground.  It  forms  the  record  of  the  slow  and 
almost  imperceptible  geological  changes  that  affect  the  regions 
where  it  accumulates, —  the  quiet  fall  of  rain,  the  gradual  rot- 
ting away  of  the  upper  part  of  the  underlying  rock,  the  growth 
and  decay  of  a  long  succession  of  generations  of  plants,  the 


•^:..^K- . 


Fig.  4. —  Talus-slopes  at  the  foot  of  a  line  of  cliffs. 

ceaseless  labours  of  the  earthworm,  the  scarcely  appreciable 
removal  of  material  from  the  surface  by  the  action  of  rain  and 
wind,  and  the  equally  insensible  descent  of  the  crumbling  subsoil 
farther  and  farther  into  the  solid  stone  below.  Having  learnt 
how  all  this  is  told  by  the  soil  beneath  our  feet,  we  should  be 
ready  to  recognise  in  the  soil  of  former  ages  a  similar  chronicle 
of  quiet  atmospheric  disintegration. 

Talus. —  Besides  soil  and  subsoil,  there  are  other  forms  in 
which  decomposed  rock  accumulates  on  the  surface  of  the  land. 
Where  a  large  mass  of  bare  rock  rises  up  as  a  steep  bank  or 
cliff,  it  is  liable  to  constant  degradation,  and  the  materials 
detached  from  its  surface  accumulate  down  the  slopes,  forming 
what  is  known  as  a  Talus  (Fig.  4).  In  mountainous  or  hilly 
regions,  where  rocky  precipices  rise  high  into  the  air,  there 


20  GEOLOGY 

gather  at  their  feet  and  down  their  clefts  long  trails  or  screes 
of  loose  blocks  that  have  been  split  off  from  them  by  the  weather. 
Such  slopes,  especially  where  they  are  not  too  steep,  and  where 
the  rubbish  that  forms  them  is  not  too  coarse,  may  be  more 
or  less  covered  with  vegetation,  which  in  some  measure  arrests 
the  descent  of  the  debris.  But  from  time  to  time,  during  heavy 
rains,  deep  gullies  are  torn  out  of  them  by  rapidly  formed 
torrents,  which  sweep  down  their  materials  to  lower  levels 
(Fig.  10).  The  sections  laid  bare  in  these  gullies  show  that 
the  rubbish  is  arranged  in  more  or  less  distinct  layers  which 
lie  generally  parallel  with  the  surface  of  the  slope;  in  other 
words,  it  is  rudely  stratified,  and  its  layers  or  strata  are  inclined 
at  the  angle  of  the  declivity  which  seldom  exceeds  35°. 

Rain-wash,  Brick-earth. —  On  more  gen- 
tle slopes,  even  where  no  bare  rock  projects 
into  the  air,  the  fall  of  rain  gradually 
washes  down  the  upper  parts  of  the  soil 
to  lower  levels.  Hence  arise  thick  accum- 
ulations of  what  is  known  as  rain-wasli  — 
soil  mixed  often  with  angular  fragments 
of  still  undecomposed  rock,  and  not  in- 
frequently forming  a  kind  of  brick-earth 
(Fig.  5).  Deposits  of  this  nature  are  still 

_,.  ..  gathering  now,  though  their  lower  portions 

Fig.     5. —  Section     of  g  to£  ° 

rain-wash  or  brick-  may  be  oi  great  antiquity.     In  the  soutn- 

Ioihh'6  7Br7ckg-faarthe  east  of  England>  for  instance,  the  brick- 

5.   White  sand.    4.  earths   contain  the  bones  of  animals  that 

W  hYtV  esand.     I.  nave  ^on§  since  passed  away. 

Brick-earth.    1.  Dust. —  By   the   action   of   wind,    above 

Gravel    with    seams  /?         n   ,                                  ±     *  n        ^              n 

of  sand.  referred  to,  a  vast  amount  oi  fine  dust  and 

sand  is  carried  up  into  the  air  and 
strewn  far  and  wide  over  the  land.  In  dry  countries,  such  as 
large  tracts  of  Central  Asia,  the  air  is  often  thick  with  a  fine 
yellow  dust  which  may  entirely  obscure  the  sun  at  mid-day, 
and  which  settles  over  everything.  After  many  centuries,  a 
deposit,  which  may  be  hundreds  of  feet  deep,  is  thus  accumulated 
on  the  surface  of  the  land.  Some  of  the  ancient  cities  of  the 
Old  World,  Nineveh  and  Babylon  for  example,  after  being  long 
abandoned  by  man,  have  gradually  been  buried  under  the  fine 
soil  drifted  over  them  by  the  wind  and  intercepted  and  protected 


INFLUENCE  OF  THE  ATMOSPHERE  21 

by  the  weeds  that  grew  up  over  the  ruins.  Even  in  regions 
where,  as  in  Britain,  there  is  a  large  annual  rainfall,  seasons 
of  drought  occur,  during  which  there  may  be  a  considerable 
drifting  of  the  finer  particles  of  soil  by  the  wind.  We  probably 
hardly  realise  how  much  the  soil  may  be  removed  here  and 
heightened  there  from  this  cause. 

Sand-dunes. —  Some  of  the  most  striking  and  familiar  ex- 
amples of  the  accumulation  of  loose  deposits  by  the  wind  are 
those  to  which  the  name  of  Dunes  is  given.  On  sandy  shores, 
exposed  to  winds  that  blow  landwards,  the  sand  is  dried  and 
then  carried  away  from  the  beach,  gathering  into  long  mounds 
or  ridges  which  run  parallel  to  the  coast-line.  These  ridges  are 
often  50  or  60  feet,  sometimes  even  more  than  250  feet  high, 
with  deep  troughs  and  irregular  circular  hollows  between  them, 
and  they  occasionally  form  a  strip  several  miles  broad,  bor- 
dering the  sea.  The  particles  of  sand  are  driven  inland  by 


Fig.  6. —  Sand-dunes. 

the  wind,  and  the  dunes  gradually  bury  fields,  roads,  and  villages, 
unless  their  progress  is  arrested  by  the  growth  of  vegetation 
over  their  shifting  surfaces.  On  many  parts  of  the  west  coast 
of  Europe,  the  dunes  are  marching  into  the  interior  at  the  rate 
of  20  feet  in  a  year.  Hence  large  tracts  of  land  have  within 
historic  times  been  entirely  lost  under  them.  In  the  north  of 
Scotland,  for  example,  an  ancient  and  extensive  barony,  so 
noted  for  its  fertility  that  it  was  called  "  the  granary  of  Moray/' 


22  GEOLOGY 

I 

was  devastated  about  the  middle  of  the  seventeenth  century  by 
the  moving  sands,  which  now  rise  in  barren  ridges  more  than 
100  feet  above  the  site  of  the  buried  land.  In  the  interior  of 
continents  also,  where  with  great  dryness  of  climate  there  is  a 
continual  disintegration  of  the  surface  of  rocks,  wide  wastes 
of  sand  accumulate,  as  in  the  deserts  of  Libya,  Arabia,  and 
Gobi,  in  the  heart  of  Australia,  and  in  many  of  the  western 
parts  of  the  United  States. 

There  can  be  no  doubt,  however,  that  though  the  layer  of 
vegetable  soil,  the  heaps  of  rubbish  that  gather  on  slopes  and 
at  the  base  of  rocky  banks  and  precipices,  and  the  widespread 
drifting  of  dust  and  sand  over  the  land,  afford  evidence  that 
much  of  the  material  arising  from  the  general  decay  of  the 
surface  of  the  land  accumulates  under  various  forms  upon  that 
surface,  nevertheless  its  stay  there  is  not  permanent.  Wind 
and  rain  are  continually  removing  it,  sometimes  in  vast  quan- 
tities, into  the  sea.  Every  brook,  made  muddy  by  heavy  rain, 
is  an  example  of  this  transport,  for  the  mud  that  discolours 
the  water  is  simply  the  finer  material  of  the  soil  washed  off 
by  rain.  When  we  reflect  upon  the  multitude  of  streams,  large 
and  small,  in  all  parts  of  the  globe,  and  consider  that  they 
are  all  busy  carrying  their  freights  of  mud  to  the  sea,  we  can 
in  some  measure  appreciate  how  great  must  be  the  total  annual 
amount  of  material  so  removed.  What  becomes  of  this  material 
will  form  the  subject  of  succeeding  chapters. 

SUMMARY. —  The  first  lesson  to  be  learnt  from  an  examination 
of  the  surface  of  the  land  is,  that  everywhere  decay  is  in  progress 
upon  it.  Wherever  the  solid  rock  rises  into  the  air,  it  breaks 
up  and  crumbles  away  under  the  various  influences  combined 
in  the  process  of  Weathering.  The  wasted  materials  caused  by 
this  universal  disintegration  partly  accumulate  where  they  are 
formed,  and  make  soil.  But  in  large  measure,  also,  they  are 
blown  away  by  wind  and  washed  off  by  rain.  Even  where  they 
appear  to  be  securely  protected  by  a  covering  of  vegetation,  the 
common  earth-worm  brings  the  finer  parts  of  them  up  to  the 
surface,  where  they  come  within  reach  of  rain  and  wind,  so  that 
on  tracts  permanently  grassed  over,  there  may  be  a  continuous 
and  not  inconsiderable  removal  of  fine  soil  from  the  surface. 
In  proportion  as  the  upper  layers  of  soil  are  removed,  roots  and 
percolating  water  are  enabled  to  reach  down  farther  into  the 


INFLUENCE  OF  THE  ATMOSPHERE  23 

solid  rock  which  is  broken  up  into  subsoil,  and  thus  the  general 
surface  of  the  land  is  insensibly  lowered. 

Besides  accumulating  in  situ  as  subsoil  and  soil,  the  debris 
of  decomposed  rock  forms  talus-slopes  and  screes  at  the  foot  of 
crags,  and  a  layer  of  rain-wash  or  brick-earth  over  gentler  slopes. 
Where  the  action  of  wind  comes  markedly  into  play,  tracts  of 
sand-dunes  may  be  piled  up  along  the  borders  of  the  sea  and 
of  lakes,  or  in  the  arid  interior  of  continents ;  and  wide  regions 
have  been  in  course  of  time  buried  under  the  fine  dust  which 
is  sometimes  so  thick  in  the  air  as  to  obscure  the  noonday  sun. 
But  in  none  of  these  forms  can  the  accumulation  of  decomposed 
material  be  regarded  as  permanent.  So  long  as  it  is  exposed 
to  the  influences  of  the  atmosphere,  this  material  is  still  liable 
to  be  swept  away  from  the  surface  of  the  land  and  borne  out- 
wards into  the  sea. 


24  GEOLOGY 


CHAPTER  III. 

INFLUENCE  OF   RUNNING   WATER   IN  GEOLOGICAL   CHANGES,   AND 
HOW  IT  IS  RECORDED. 

IT  appears,  then,  that  from  various  causes  all  over  the  globe, 
there  is  a  continual  decay  of  the  surface  of  the  land;  that 
the  decomposed  material  partly  accumulates  as  soil,  subsoil, 
and  sheets  or  heaps  of  loose  earth  or  sand,  but  that  much  of  it 
is  washed  off  the  land  by  rain  or  blown  into  the  rivers  or  into 
the  sea  by  wind.  We  have  now  to  consider  the  part  taken  by 
Running  Water  in  this  transport.  From  the  single  rain-drop 
up  to  the  mighty  river,  every  portion  of  the  water  that  flows 
over  the  land  is  busy  with  its  own  share  of  the  work.  When 
we  reflect  on  the  amount  of  rain  that  falls  annually  over  the 
land,  and  on  the  number  of  streams,  large,  and  small,  that  are 
ceaselessly  at  work,  we  realise  how  difficult  it  must  be  to  form 
any  fit  notion  of  the  entire  amount  of  change  which,  even  in 
a  single  year,  these  agents  work  upon  the  surface  of  the  earth. 

The  influence  of  rain  in  the  decay  of  the  surface  of  the  land 
was  briefly  alluded  to  in  the  last  chapter.  As  soon  as  a  drop 
of  rain  reaches  the  ground,  it  begins  its  appointed  geological 
task,  dissolving  what  it  can  carry  off  in  solution,  and  pushing 
forward  and  downward  whatever  it  has  power  to  move.  As  the 
rain-drops  gather  into  runnels,  the  same  duty,  but  on  a  larger 
scale,  is  performed  by  them;  and  as  the  runnels  unite  into 
large  streams,  and  these  into  yet  mightier  rivers,  the  operations, 
though  becoming  colossal  in  magnitude,  remain  essentially  the 
same  in  kind.  In  the  operations  of  the  nearest  brook,  we  see 
before  us  in  miniature  a  sample  of  the  changes  produced  by 
the  thousands  of  rivers  which,  in  all  quarters  of  the  globe,  are 
flowing  from  the  mountains  to  the  sea.  Watching  these  opera- 
tions from  day  to  day,  we  discover  that  they  may  all  be  classed 
under  two  heads.  In  the  first  place,  the  brook  hollows  out  the 


INFLUENCE  OF  RUNNING  WATER  25 

channel  in  which  it  flows  and  thus  aids  in  the  general  waste 
of  the  surface  of  the  land;  and  in  the  second  place,  it  carries 
away  fine  silt  and  other  material  resulting  from  that  waste, 
and  either  deposits  it  again  on  the  land  or  carries  it  out  to  sea. 
Rivers  are  thus  at  once  agents  that  themselves  directly  degrade 
the  land,  and  that  sweep  the  loosened  detritus  towards  the 
ocean.  An  acquaintance  with  each  of  these  kinds  of  work  is 
needful  to  enable  us  to  understand  the  nature  of  the  records 
which  river-action  leaves  behind  it. 

i.  EROSIVE  AND  TRANSPORTING  POWER  OF  RUNNING  WATER. 

CHEMICAL  ACTION. —  We  have  seen  that  rain  in  its  descent 
from  the  clouds  absorbs  air,  and  that  with  the  oxygen  and 
carbonic  acid  which  it  thus  obtains  it  proceeds  to  corrode  the 
surfaces  of  rock  on  which  it  falls.  When  it  reaches  the  ground 
and  absorbs  the  acids  termed  "humous,"  which  are  supplied 
by  the  decomposing  vegetation  of  the  soil,  it  acquires  increased 
power  of  eating  into  the  stones  over  which  it  flows.  When  it 
rolls  along  as  a  runnel,  brook,  or  river,  it  no  doubt  still  attacks 
the  rocks  of  its  channel,  though  its  action  in  this  respect  is  not 
so  easily  detected.  In  some  circumstances,  however,  the  solvent 
influence  of  river-water  upon  solid  rocks  is  strikingly  displayed. 
Where  the  water  contains  a  large  proportion  of  the  acids  of 
the  soil,  and  flows  over  a  kind  of  rock  specially  liable  to  be 
eaten  away  by  these  acids,  the  most  favourable  conditions  are 
presented  for  observing  the  change.  Thus,  a  stream  which 
issues  from  a  peat-bog  is  usually  dark  brown  in  colour,  from 
the  vegetable  solutions  which  it  extracts  from  the  moss.  Among 
these  solutions  are  some  of  the  organic  acids  referred  to,  ready 
to  eat  into  the  surface  of  the  rocks  or  loose  stones  which  the 
stream  may  encounter  in  its  descent.  No  kind  of  rock  is  more 
liable  than  limestone  to  corrosion  under  such  circumstances. 
Peaty  water  flowing  over  it  eats  it  away  with  comparative 
rapidity,  while  those  portions  of  the  rock  that  rise  above  the 
stream  escape  solution,  except  in  so  far  as  they  are  attacked 
by  rain.  Hence  arise  some  curious  features  in  the  scenery  of 
limestone  districts.  The  walls  of  limestone  above  the  water, 
being  attacked  only  by  the  atmosphere,  are  not  eaten  away  so 
fast  as  their  base,  over  which  the  stream  flows.  They  are  con- 


26  GEOLOGY 

sequently  undermined,  and  are  sometimes  cut  into  dark  tunnels 
and  passages  (Fig.  7).  Even  where  the  solvent  action  of  the 
water  of  rivers  is  otherwise  inappreciable,  it  can  be  detected  by 
means  of  chemical  analysis.  Thus  rivers,  partly  by  the  action 
of  their  water  upon  the  loose  stones  and  solid  rocks  of  their 
channels,  and  parti}  b}  th'  contributions  they  receive  from 
Spring!:  (which  will  be  afterward;  described),  convey  a  vast 
amount  of  dissolved  material  into  the  sea.  The  mineral  sub- 
stance thus  invisibly  transported  consists  of  various  salts.  One 
of  the  most  abundant  of  these  —  carbonate  of  lime  —  is  the 
substance  that  form0  limestone,  and  furnishes  the  mineral  mat- 
ter required  for  the  hard  parts  of  a  large  proportion  of  the 
lower  animals.  It  is  a  matter  of  some  interest  to  know  that 


Fig.  7. —  Erosion  of  limestone  by  the  solvent  action  of  a  peaty  stream, 
Durness,   Sutherlandshire. 

this  substance,  so  indispensable  for  the  formation  of  the  shells 
of  so  large  a  number  of  sea-creatures,  is  constantly  supplied 
to  the  sea  by  the  streams  that  flow  into  it.1  The  rivers  of 
Western  Europe,  for  instance,  ha.ve  been  ascertained  to  convey 
about  1  part  of  dissolved  mineral  matter  in  every  5000  parts 
of  water,  and  of  this  mineral  matter  about  a  half  consists  of 
carbonate  of  lime.  It  has  been  estimated  that  the  Ehine  bears 

1  There  is  now  reason,  however,  to  suspect  that  the  carbonate  of  lime 
in  marine  organisms  is  not  derived  so  much  from  the  comparatively 
minute  proportion  of  that  substance  present  in  solution  in  sea-water, 
as  from  the  much  more  abundant  sulphate  of  lime  which  undergoes 
apparently  a  process  of  chemical  transformation  into  carbonate  within 
the  living  animals. 


INFLUENCE  OF  RUNNING  WATER  27 

enough  carbonate  of  lime  into  the  sea  every  year  to  make  three 
hundred  and  thirty-two  thousand  millions  of  oysters  of  the 
usual  size.  Another  abundant  ingredient  of  river-water  is 
gypsum  or  sulphate  of  lime,  of  which  the  Thames  is  computed 
to  carry  annually  past  London  not  less  than  180,000  tons.  The 
total  quantity  of  carbonate  of  lime,  removed  from  the  limestones 
of  its  basin  by  this  river  in  a  year,  amounts,  on  an  average, 
to  140  tons  from  every  square  mile,  which  is  estimated  to  be 
equal  to  the  lowering  of  the  general  surface  to  the  extent 
of  dfo  of  an  inch  from  each  square  mile  in  a  century,  or  one  foot 
in  13,200  years. 

MECHANICAL  ACTION. — (1)  Transport. —  The  dissolved  ma- 
terial forms  but  a  small  proportion  of  the  total  amount  of 
mineral  substances  conveyed  by  rivers  from  land  to  sea.  A 
single  shower  of  rain  washes  off  fine  dust  and  soil  from  the 
surface  of  the  ground  into  the  nearest  brook  which  then  rolls 
along  with  a  discoloured  current.  An  increase  in  the  volume 
of  the  water  enables  a  stream  to  sweep  along  sand,  gravel,  and 
blocks  of  stone  lying  in  its  channel,  and  to  keep  these  materials 
moving  until,  as  the  declivity  lessens  and  the  rain  ceases,  the 
current  becomes  too  feeble  to  do  more  than  lazily  carry  onward 
the  fine  silt  that  discolours  it.  Every  stream,  large  or  small,  is 
ceaselessly  busy  transporting  mud,  sand,  or  gravel.  And  as  the 
ultimate  destination  of  all  this  sediment  is  the  bottom  of  the 
sea,  it  is  evident  that  if  there  be  no  compensating  influences 
at  work  to  repair  the  constant  loss,  the  land  must  in  the  end 
be  all  worn  away. 

Some  of  the  most  instructive  lessons  regarding  the  work  of 
running  water  on  land  are  afforded  by  the  beds  of  mountain- 
torrents.  Huge  blocks,  detached  from  the  crags  and  cliffs  on 
either  side,  may  there  be  seen  cumbering  the  pathway  of  the 
water,  which  seems  quite  powerless  to  move  such  masses  and 
can  only  sweep  round  them  or  find  a  passage  beneath  them. 
But  visit  such  a  torrent  when  it  is  swollen  with  heavy  rains 
or  rapidly  melted  snow,  and  you  will  hear  the  stones  knocking 
against  each  other  or  on  the  rocky  bottom,  as  they  are  driven 
downwards  by  the  flood.  Or  when  the  stream  is  at  its  lowest, 
in  dry  summer  weather,  follow  its  course  a  little  way  down  hill, 
and  you  will  see  that  by  degrees  the  blocks,  losing  their  sharp 
edges,  have  become  rounded  boulders,  and  that  these  are  gradu- 


28  GEOLOGY 

ally  replaced  by  coarse  shingle  and  then  by  finer  gravel.  In 
the  quieter  reaches  of  the  water,  sheets  of  sand  begin  to  make 
their  appearance,  and  at  last  when  the  stream  reaches  the  plains, 
no  sediment  of  coarser  grain  than  mere  silt  may  be  seen  in 
its  channel.  It  is  thus  obvious  that  in  the  constant  transport 
maintained  by  watercourses,  the  carried  materials,  by  being  rolled 
along  rocky  channels  and  continually  ground  against  each  other, 
diminish  in  size  as  they  descend.  A  river  flowing  from  a  range 
of  mountains  to  the  distant  ocean  may  be  likened  to  a  mill, 
into  which  large  angular  masses  of  rock  are  cast  at  the  upper 
end,  and  out  of  which  only  fine  sand  and  silt  are  discharged  at 
the  lower. 

Partly,  therefore,  owing  to  the  fine  dust  and  soil  swept  into 
them  by  wind  and  rain  from  the  slowly  decomposing  surface 
of  the  land,  and  partly  to  the  friction  of  the  detritus  which 
they  sweep  along  their  channels,  rivers  always  contain  more 
or  less  mineral  matter  suspended  in  their  water  or  travelling 
with  the  current  on  the  bottom.  The  amount  of  material  thus 
transported  varies  greatly  in  different  rivers,  and  at  successive 
seasons  even  in  the  same  river.  In  some  cases,  the  rainfall  is 
spread  so  equably  through  the  year  that  the  rivers  flow  onward 
with  a  quiet  monotony,  never  rising  much  above  nor  sinking 
much  below  their  average  level.  In  such  circumstances,  the 
amount  of  sediment  they  carry  downward  is  proportionately 
small.  On  the  other  hand,  where  either  from  heavy  periodical 
rains  or  from  rapid  melting  of  snow,  rivers  are  liable  to  floods, 
they  acquire  an  enormously  increased  power  of  transport,  and 
their  burden  of  sediment  is  proportionately  augmented.  In  a 
few  days  or  weeks  of  high  water,  they  may  convey  to  the  sea  a 
hundredfold  the  amount  of  mineral  matter  which  they  could 
carry  in  a  whole  year  of  their  quieter  mood. 

Measurements  have  been  made  of  the  proportions  of  sediment 
in  the  waters  of  different  rivers  at  various  seasons  of  the  year. 
The  results,  as  might  be  expected,  show  great  variations.  Thus 
the  Garonne,  rising  among  the  higher  peaks  of  the  Pyrenees, 
drains  a  large  area  of  the  south  of  France,  and  is  subject  to  floods 
by  which  an  enormous  quantity  of  sediment  is  swept  down  from 
the  mountains  to  the  plains.  Its  proportion  of  mud  has  been 
estimated  to  be  as  much  as  1  part  in  100  parts  of  water.  The 
Durance,  which  takes  its  source  high  on  the  western  flank  of  the 


INFLUENCE  OF  RUNNING  WATER  29 

Cottian  Alps,  is  one  of  the  rapidest  and  muddiest  rivers  in 
Europe.  Its  angle  of  slope  varies  from  1  in  208  to  1  in  467,  the 
average  declivity  of  the  great  rivers  of  the-  globe  being  probably 
not  more  than  1  in  2600,  while  that  of  a  navigable  stream  ought 
not  to  exceed  10  inches  per  mile  or  1  in  6336.  The  Durance  is, 
therefore,  rather  a  torrent  than  a  river.  With  this  rapidity  of 
descent  is  conjoined  an  excessive  capacity  for  transporting  sedi- 
ment. In  floods  of  exceptional  severity,  the  proportion  of  mud 
in  the  stream  has  been  estimated  at  one-tenth  by  weight  of  the 
water,  while  the  average  proportion  for  nine  years  from  1867 
to  1875  was  about  -sh.  Probably  the  best  general  average  is 
to  be  obtained  from  a  river  which  drains  a  wide  region  exhibiting 
considerable  diversities  of  climate,  topography,  rocks,  and  soils. 
The  Mississippi  presents  a  good  illustration  of  these  diversities, 
and  has  accordingly  been  taken  as  a  kind  of  typical  river,  fur- 
nishing, so  to  speak,  a  standard  by  which  the  operations  of  other 
rivers  may  be  compared,  and  which  may  perhaps  be  assumed  as 
a  fair  average  for  all  the  rivers  of  the  globe.  Numerous  meas- 
urements have  been  made  of  the  proportion  of  sediment  carried 
into  the  Gulf  of  Mexico  by  this  vast  river,  with  the  result  of 
showing  that  the  average  amount  of  sediment  is  by  weight  1  part 
in  every  1500  parts  of  water,  or  little  more  than  one-third  of  the 
proportion  in  the  water  of  the  Durance. 

If  now  we  assume  that,  all  over  the  world,  the  general  average 
proportion  of  sediment  floating  in  the  water  of  rivers  is  1  part  in 
every  1500  of  water,  we  can  readily  understand  how  seriously  in 
the  course  of  time  must  the  land  be  lowered  by  the  constant 
removal  of  so  much  decomposed  rock  from  its  surface.  Knowing 
the  area  of  the  basin  drained  by  a  river,  and  also  the  proportion 
of  sediment  in  its  water,  we  can  easily  calculate  the  general  loss 
from  the  surface  of  the  basin.  The  ratio  of  the  weight  or 
"  specific  gravity  "  of  the  silt  to  that  of  solid  rock  may  be  taken 
to  be  as  19  is  to  25.  Accordingly  the  Mississippi  conveys  annually 
from  its  drainage  basin  an  amount  of  sediment  equivalent  to  the 
removal  of  FoW  part  of  a  foot  of  rock  from  the  general  sur- 
face of  the  basin.  At  this  rate,  one  foot  of  rock  will  be  worn 
away  every  6000  years.  If  we  take  the  general  height  of  the 
land  of  the  whole  globe  to  be  2120  feet,  and  suppose  it  to  be  con- 
tinuously wasted  at  the  same  rate  at  which  the  Mississippi  basin 
is  now  suffering,  then  the  whole  dry  land  would  be  carried  into 


30  GEOLOGY 

the  sea  in  12,720,000  years.  Or  if  we  assume  the  mean  height  of 
Europe  to  be  973  feet  and  that  this  continent  is  degraded  at  the 
Mississippi  rate  of  waste  until  the  last  vestige  of  it  disappears, 
the  process  of  destruction  would  be  completed  in  rather  less  than 
6,000,000  years.  Such  estimates  are  not  intended  to  be  close 
approximations  to  the  truth.  As  the  land  is  lowered,  the  rate 
of  decay  will  gradually  diminish,  so  that  the  later  stages  of  decay 
will  be  enormously  protracted.  But  by  taking  the  rate  of  opera- 
tion now  ascertained  to  be  in  progress  in  such  a  river  basin  as  the 
Mississippi,  we  obtain  a  valuable  standard  of  comparison,  and 
learn  that  the  degradation  of  the  land  is  much  greater  and  more 
rapid  than  might  have  been  supposed. 

(2)  Erosion. — But  rivers  are  not  merely  carriers  of  the  mud, 
sand,  and  gravel  swept  into  their  channels  by  other  agencies.  By 
keeping  these  materials  in  motion,  the  currents  reduce  them  in 
size,  and  at  the  same  time  employ  them  to  hollow  out  the  chan- 
nels wherein  they  move.  The  mutual  friction  that  grinds  down 
large  blocks  of  rock  into  sand  and  mud,  tells  also  upon  the  rocky 
beds  along  which  the  material  is  driven.  The  most  solid  rocks 
are  worn  down;  deep  long  gorges  are  dug  out,  and  the  water- 
courses, when  they  have  once  chosen  their  sites,  remain  on  them 
and  sink  gradually  deeper  and  deeper  beneath  the  general  level 
of  the  country.  The  surfaces  of  stone  exposed  to  this  attrition 
assume  the  familiar  smoothed  and  rounded  appearance  which  is 
known  as  water-worn.  The  loose  stones  lying  in  the  channel  of 
a  stream,  and  the  solid  rocks  as  high  up  as  floods  can  scour  them, 
present  this  characteristic  aspect.  Here  and  there,  where  a  few 
stones  have  been  caught  in  an  eddy  of  the  current,  and  are  kept 
in  constant  gyration,  they  reduce  each  other  in  dimensions,  and 
at  the  same  time  grind  out  a  hollow  in  the  underlying  rock.  The 
sand  and  mud  produced  by  the  friction  are  swept  off  by  the 
current,  and  the  stones  when  sufficiently  reduced  in  size  are  also 
carried  away.  But  their  places  are  eventually  taken  by  other 
blocks  brought  down  by  floods,  so  that  the  supply  of  grinding 
material  is  kept  up  until  the  original  hollow  is  enlarged  into  a 
wide  deep  caldron,  at  the  bottom  of  which  the  stones  can  only 
be  stirred  by  the  heaviest  floods.  Cavities  of  this  kind,  known  as 
pot-holes,  are  of  frequent  occurrence  in  rocky  watercourses  as 
well  as  on  rocky  shores,  in  short,  wherever  eddies  of  water  can 
keep  shingle  rotating  upon  solid  rock.  As  they  often  coalesce 


INFLUENCE  OF  RUNNING  WATER 


31 


by  the  wearing  away  of  the  intervening  wall  of  rock,  they  greatly 
aid  in  the  deepening  of  a  watercourse.  In  most  rocky  gorges,  a 
succession  of  old  pot-holes  may  be  traced  far  above  the  present 
level  of  the  stream  (Fig.  8). 

That  it  is  by  means  of  the  gravel  and  other  detritus  pushed 
along  the  bottom  by  the  current,  rather  than  by  the  mere  friction 
of  the  water  on  its  bed,  that  a  river  excavates  its  channel,  is  most 
strikingly  shown  immediately  below  a  lake.  In  traversing  a  lake^ 


Fig.  8. —  Pot-holes  worn  out  by  the  gyration  of  stones  in  the  bed  of  a 

stream. 

the  tributary  streams  deposit  their  sediment  on  its  bottom,  be- 
cause the  still  water  checks  their  current  and,  by  depriving  the 
water  of  its  more  rapid  movement,  compels  it  to  drop  its  burden 
of  gravel,  sand,  and  silt  (see  Lakes).  Filtered  in  this  way,  the 
various  streams  united  in  the  lake  escape  at  its  lower  end  as  a 
clear  transparent  river.  The  Khone,  for  instance,  flows  into  the 
Lake  of  Geneva  as  a  turbid  stream;  it  issues  from  that  great 
reservoir  at  Geneva  as  a  rushing  current  of  the  bluest,  most 
translucent  water  which,  though  it  sweeps  over  ledges  of  rock, 
has  not  yet  been  able  to  grind  them  down  into  a  deep  gorge.  The 


32  GEOLOGY 

Niagara,  also,  filtered  by  Lake  Erie,  has  not  acquired  sediment 
enough  to  enable  it  to  cut  deeply  into  the  rocks  over  which  it 
foams  in  its  rapids  before  throwing  itself  over  the  great  Falls. 

One  of  the  most  characteristic  features  of  streams  is  the 
singularly  sinuous  courses  they  follow.  As  a  rule,  too,  the  natter 
the  ground  over  which  they  flow,  the  more  do  they  wind.  Not 
uncommonly  they  form  loops,  the  nearest  bends  of  which  in  the 
end  unite,  and  as  the  current  passes  along  the  now  straightened 
channel,  the  old  one  is  left  to  become  by  degrees  a  lake  or  pond 
of  stagnant  water,  then  a  marsh,  and  lastly,  dry  ground.  We 
might  suppose  that  in  flowing  off  the  land,  water  would  take  the 
shortest  and  most  direct  road  to  the  sea.  But  this  is  far  from 
being  the  case.  The  slightest  inequalities  of  level  have  originally 
determined  sinuosities  of  the  channels,  while  trifling  differences 
in  the  hardness  of  the  banks,  in  the  accumulation  of  sediment, 
and  in  the  direction  of  the  currents  and  eddies,  have  been  enough 
to  turn  a  stream  now  to  one  side  now  to  another,  until  it  has 
assumed  its  present  meandering  course.  How  easily  this  may  be 
done  can  be  instructively  observed  on  a  roadway  or  other  bare 
surface  of  ground.  When  quite  dry  and  smooth,  hardly  any  de- 
pressions in  which  water  would  flow  might  be  detected  on  such 
a  surface.  But  after  a  heavy  shower  of  rain,  runnels  of  muddy 
water  will  be  seen  coursing  down  the  slope  in  serpentine  chan- 
nels that  at  once  recall  the  winding  rivers  of  a  great  drainage- 
system.  The  slightest  differences  of  level  have  been  enough  to 
turn  the  water  from  side  to  side.  A  mere  pebble  or  projecting 
heap  of  earth  or  tuft  of  grass  has  sufficed  to  cause  a  bend.  The 
water,  though  always  descending,  has  only  been  able  to  reach 
the  bottom  by  keeping  the  lowest  levels,  and  turning  from  right 
to  left  as  these  guided  it. 

When  a  river  has  once  taken  its  course  and  has  begun  to  ex- 
cavate its  channel,  only  some  great  disturbance,  such  as  an 
earthquake  or  volcanic  eruption,  can  turn  it  out  of  that  course. 
If  its  original  pathway  has  been  a  winding  one,  it  goes  on  dig- 
ging out  its  bed  which,  with  all  its  bends,  gradually  sinks  below 
the  level  of  the  surrounding  country.  The  deep  and  pictur- 
esque gorge  in  which  the  Moselle  winds  from  Treves  to  Coblenz 
has  in  this  way  been  slowly  eroded  out  of  the  undulating  table- 
land across  which  the  river  originally  flowed. 

In  another  and  most   characteristic  way,  the  shape  of  the 


INFLUENCE  OF  RUNNING  WATER 


33 


34  GEOLOGY 

ground  and  the  nature  and  arrangement  of  the  rocks  over  which 
they  flow,  materially  influence  rivers  in  the  forms  into  which 
they  carve  their  channels.  The  Rhone  and  the  Niagara,  for 
instance,  though  filtered  by  the  lakes  through  which  they  flow, 
do  not  run  far  before  plunging  into  deep  ravines.  Obviously 
such  ravines  cannot  have  been  dug  out  by  the  same  process  of 
mechanical  attrition  whereby  river-channels  in  general  are 
eroded.  Yet  the  frequency  of  gorges  in  river  scenery  shows  that 
they  cannot  be  due  to  any  exceptional  operation.  They  may 
generally  be  accounted  for  by  some  arrangement  of  rocks  wherein 
a  bed  of  harder  material  is  underlain  by  one  more  easily  re- 
movable. Where  a  stream,  after  flowing  over  the  upper  bed, 
encounters  the  decomposable  bed  below,  it  eats  away  the  latter 
more  rapidly.  The  overlying  hard  rock  is  thus  undermined, 
and,  as  its  support  is  destroyed,  slice  after  slice  is  cut  away 
from  it.  The  waterfall  which  this  kind  of  structure  produces 
continues  to  eat  its  way  backward  or  up  the  course  of  the  stream, 
so  long  as  the  necessary  conditions  are  maintained  of  hard  rocks 
lying  upon  soft.  Any  change  of  structure  which  would  bring 
the  hard  rocks  down  to  the  bed  of  the  channel,  and  remove  the 
soft  rocks  from  the  action  of  the  current  and  the  dash  of  the 
spray,  would  gradually  destroy  the  waterfall.  It  is  obvious, 
that,  by  cutting  its  way  backward,  a  waterfall  excavates  a  ravine. 
The  renowned  Falls  of  Niagara  supply  a  striking  illustration 
of  the  process  now  described.  The  vast  body  of  water  which 
issues  from  Lake  Erie,  after  flowing  through  a  level  country 
for  a  few  miles,  rushes  down  its  rapids  and  then  plunges  over  a 
precipice  of  solid  limestone.  Beneath  this  hard  rock  lies  a 
band  of  comparatively  easily  eroded  shale.  As  the  water  loosens 
and  removes  the  lower  rock,  large  portions  of  the  face  of  the 
precipice  behind  the  Falls  are  from  time  to  time  precipitated 
into  the  boiling  flood  below.  The  waterfall  is  thus  slowly  pro- 
longing the  ravine  below  the  Falls.  The  magnificent  gorge  in 
which  the  Niagara,  after  its  tumultuous  descent,  flows  sullenly  to 
Lake  Ontario  is  not  less  than  7  miles  long,  from  200  to  400 
yards  wide,  and  from  200  to  300  feet  deep.  There  is  no  reason 
to  doubt  that  this  chasm  has  been  entirely  dug  out  by  the  grad- 
ual recession  of  the  Falls  from  the  cliffs  at  Queenston,  over 
which  the  river  at  first  poured.  We  may  form  some  conception 
of  the  amount  of  rock  thus  removed  from  the  estimate  that  it 


INFLUENCE  OF  RUNNING  WATER  35 

would  make  a  rampart  about  12  feet  high  and  6  feet  thick  ex- 
tending right  round  the  whole  globe  at  the  equator.  Still  more 
gigantic  are  the  gorges  or  canons  of  the  Colorado  and  its  tribu- 
taries in  Western  America.  The  Grand  Canon  of  the  Colorado 
is  300  miles  long,  and  in  some  places  more  than  6000  feet  deep 
(Fig.  9).  The  country  traversed  by  it  is  a  network  of  profound 
ravines,  at  the  bottom  of  which  the  streams  flow  that  have  eroded 
them  out- of  the  table-land. 

ii.    DEPOSITION  OF  MATERIALS  BY  RUNNING  WATER. 

Permanent  Records  of  River- Action. —  If,  then,  all  the 
streams  on  the  surface  of  the  globe  are  engaged  in  the  double 
task  of  digging  out  their  channels  and  carrying  away  the  loose 
materials  that  arise  from  the  decomposition  of  the  surface  of 
the  land,  let  us  ask  ourselves  what  memorials  of  these  operations 
they  leave  behind  them.  In  what  form  do  the  running  waters 
of  the  land  inscribe  their  annals  in  geological  history  ?  If  these 
waters  could  suddenly  be  dried  up  all  over  the  earth,  how  could 
we  tell  what  changes  they  had  once  worked  upon  the  surface  of 
the  land  ?  Can  we  detect  the  traces  of  ancient  rivers  where  there 
are  no  rivers  now? 

From  what  has  been  said  in  this  lesson  it  will  be  evident  that 
in  answer  to  such  questions  as  these,  we  may  affirm  that  one  un- 
mistakable evidence  of  the  former  presence  of  rivers  is  to  be 
found  in  the  channels  which  they  have  eroded.  The  gorges, 
rocky  denies,  pot-holes,  and  water-worn  rocks  which  mark  the 
pathway  of  a  stream  would  long  remain  as  striking  memorials 
of  the  work  of  running  water.  In  districts,  now  dry  and  bar- 
ren, such  as  large  regions  in  the  Levant,  there  are  abundant 
channels  (wadies)  now  seldom  or  never  occupied  by  a  stream, 
but  which  were  evidently  at  one  time  the  beds  of  active  torrents. 

Alluvium. —  But  more  universal  testimony  to  the  work  of 
running  water  is  to  be  found  in  the  deposits  which  it  has  accumu- 
lated. To  these  deposits  the  general  name  of  alluvium  has  been 
given.  Spreading  out  on  either  side,  sometimes  far  beyond  the 
limits  of  the  ordinary  or  modern  channels,  these  deposits,  even 
when  worn  into  fragmentary  patches,  retain  their  clear  record  of 
the  operations  of  the  river.  Let  us  in  imagination  follow  the 
course  of  a  river  from  the  mountains  to  the  sea,  and  mark  as 


36  GEOLOGY 

we  go  the  circumstances  under  which  the  accumulation  of  sedi- 
ment takes  place. 

The  power  possessed  by  running  water  to  carry  forward  sedi- 
ment depends  mainly  upon  the  velocity  of  the  current.  The 
more  rapidly  a  stream  flows,  the  more  sediment  can  it  transport, 
and  the  larger  are  the  blocks  which  it  can  move.  The  velocity  is 
regulated  chiefly  by  the  angle  of  slope ;  the  greater  the  declivity, 
the  higher  the  velocity  and  the  larger  the  capacity  of  the  stream 


Fig.  10. —  Gullies  torn  out  of  the  side  of  a  mountain  by  descending  tor- 
rents, with  cones  of  detritus  at  their  base. 

to  carry  down  debris.  Any  cause,  therefore,  which  lessens  the 
velocity  of  a  current  diminishes  its  carrying  power.  If  water, 
bearing  along  gravel,  sand,  or  mud,  is  checked  in  its  flow,  some 
of  these  materials  will  drop  and  remain  at  rest  on  the  bottom. 
In  the  course  of  every  stream,  various  conditions  arise  whereby 
the  velocity  of  the  current  is  reduced.  One  of  the  most  obvious 
of  these  is  a  diminution  in  the  slope  of  the  channel.  Another  is 
the  union  of  a  rapid  tributary  with  a  more  gently  flowing  stream. 


INFLUENCE  OP  RUNNING  WATER  37 

A  third  is  the  junction  of  a  stream  with  the  still  waters  of  a  lake 
(see  Lakes)  or  with  the  sea.  In  these  circumstances,  the  flow  of 
the  water  being  checked,  the  sediment  at  once  begins  to  fall  to 
the  bottom. 

Tracing  now  the  progress  of  a  river,  for  illustrations  of  this 
law  of  deposition,  we  find  that  among  the  mountains  where  the 
river  takes  its  rise,  the  torrents  that  rush  down  the  declivities 
have  torn  out  of  them  such  vast  quantities  of  soil  and  rock  as  to 
seam  them  with  deep  clefts  and  gullies.  Where  each  of  these 
rapid  streamlets  reaches  the  valley  below,  its  rapidity  of  motion 
is  at  once  lessened,  and  with  this  slackening  of  speed  and  conse- 
quent loss  of  carrying  power,  there  is  an  accompanying  deposit  of 
detritus.  Blocks  of  rock,  angular  rubbish,  rounded  shingle, 
sand,  and  earth  are  thrown  down  in  the  form  of  a  cone  of  which 
the  apex  starts  from  the  bottom  of  the  gully  and  the  base  spreads 
out  over  the  plain  (Fig.  10) .  Such  cones  vary  in  dimensions  ac- 
cording to  the  size  of  the  torrent  and  the  comparative  ease  with 
which  the  rocks  of  the  mountain-side  can  be  loosened  and  re- 
moved. Some  of  them,  thrown  down  by  the  transient  runnels 
of  the  last  sudden  rain-storm,  may  not  be  more  than  a  few  cubic 
yards  in  bulk.  But  on  the  skirts  of  mountainous  regions  they 
may  grow  into  masses  hundreds  of  feet  thick  and  many  miles  in 
diameter.  The  valleys  in  a  range  of  mountains  afford  many 
striking  examples  of  these  alluvial  cones  or  fans,  as  they  are 
called.  Where  the  tributary  torrents  are  numerous,  a  succession 
of  such  cones  or  fans,  nearly  or  quite  touching  each  other,  spreads 
over  the  floor  of  a  valley.  From  this  cause,  so  large  an  amount 
of  detritus  has  within  historic  times  been  swept  down  into  some 
of  the  valleys  of  the  Tyrol  that  churches  and  other  buildings  are 
now  half -buried  in  the  accumulation. 

Looking  more  closely  at  the  materials  brought  down  by  the 
torrents,  we  find  them  arranged  in  rude  irregular  layers,  sloping 
downwards  into  the  plain,  the  coarsest  and  most  angular  detritus 
lying  nearest  to  the  mountains,  while  more  rounded  and  water- 
worn  shingle  or  sand  extends  to  the  outer  margin  of  the  cone. 
This  grouping  of  irregular  layers  of  angular  and  half-rounded 
detritus  is  characteristic  of  the  action  of  torrents.  Hence,  where 
it  occurs,  even  though  no  water  may  run  there  at  the  present  day, 
it  may  be  regarded  as  indicating  that  at  some  former  time  a 
torrent  swept  down  detritus  over  that  site. 


38  GEOLOGY 

Quitting  the  more  abrupt  declivities,  and  augmented  by  numer- 
ous tributaries  from  either  side,  the  stream  whose  course  we 
are  tracing  loses  the  character  of  a  torrent  and  assumes  that  of  a 
river.  It  still  flows  with  velocity  enough  to  carry  along  not  only 
mud  and  sand,  but  even  somewhat  coarse  gravel.  The  large 
angular  blocks  of  the  torrential  part  of  its  course,  however,  are 
no  longer  to  be  seen,  and  all  the  detritus  becomes  more  and 
more  rounded  and  smoothed  as  we  follow  it  towards  the  plains. 
At  many  places,  deposits  of  gravel  or  sand  take  place,  more 
especially  at  the  inner  side  of  the  curves  which  the  stream  makes 
as  it  winds  down  the  valley.  Sweeping  with  a  more  rapid  flow 
round  the  outer  side  of  each  curve,  the  current  lingers  in  eddies 
on  the  inner  side  and  drops  there  a  quantity  of  sediment. 
When  the  water  is  low,  strips  of  bare  sand  and  shingle  on  the 
concave  side  of  each  bend  of  the  stream  form  a  distinctive  feat- 
ure in  river  scenery.  It  is  interesting  to  walk  along  one  of  these 
strips  and  to  mark  how  the  current  has  left  its  record  there. 
The  stones  are  well  smoothed  and  rounded,  showing  that  they 
have  been  rolled  far  enough  along  the  bottom  of  the  channel 
to  lose  their  original  sharp  edges,  and  to  pass  from  the  state  of 
rough  angular  detritus  into  that  of  thoroughly  water-worn 
gravel.  Further,  they  will  be  found  not  to  lie  entirely  at  random, 
as  might  at  first  sight  be  imagined.  A  little  examination  will 
show  that,  where  the  stones  are  oblong,  they  are  generally  placed 
with  their  longer  axis  pointing  across  the  stream.  This  would 
naturally  be  the  position  which  they  would  assume  where  the 
current  kept  rolling  them  forward  along  the  channel.  Those 
which  are  flat  in  shape  will  be  observed  usually  to  slope  up 
stream.  That  the  sloping  face  must  look  in  the  direction  from 
which  the  current  moves  will  be  evident  from  Fig.  11,  where 

a  current,  moving  in  the 
direction    of    the    arrow 

and  gradually  dimmish- 
Fig.  11. —  Flat  stones  in  a  bank  of  river-   •         •      j?  T  T 
shingle,    showing    the    direction    of    the  lng    m    ±orce,    would    no 
current    (indicated   by   the  arrow)    that  longer  be  able  to  overturn 
transported  and  left  them. 

the  stones  which  it  had 

so  placed  as  to  offer  the  least  obstacle  to  its  passage.  Had  the 
current  flowed  from  the  opposite  quarter,  it  would  have  found 
the  upturned  edges  of  the  stones  exposed  to  it,  and  would  have 
readily  overturned  them  until  they  found  a  position  in  which 


INFLUENCE  OF  RUNNING  WATER 


39 


they  again  presented  least  resistance  to  the  water.  In  a  section 
of  gravel,  it  is  thus  often  quite  possible  to  tell  from  what  quarter 
the  current  flowed  that  deposited  the  pebbles. 

Yet  another  feature  in  the  arrangement  of  the  materials  is 
well  seen  where  a  digging  has  been  made  in  one  of  the  alluvial 
banks,  but  better  still  in  a  section  of  one  of  the  terraces  to  be  im- 
mediately referred  to.  The  layers  of  gravel  or  sand  in  some 
bands  may  be  observed  to  be  inclined  at  a  steeper  angle  than  in 
others,  as  shown  in  the  accompanying  figure  (Fig.  12).  In 
such  cases,  it  will  be  noticed 
that  the  slope  of  the  more  in- 
clined layers  is  down  the  stream, 
and  hence  that  their  direction 
gives  a  clue  to  that  of  the  cur- 
rent which  arranged  them.  We 
may  watch  similar  layers  in  the 
act  of  deposition  among  shallow 
pools  into  which  currents  are 
discharging  sediment.  The 

gravel  or  sand  may  be  observed 

,  ,,    J,    ,,  ,  Fig.      12. — Section      of      alluvium 

moving  along  the  bottom,  and     showing  direction  of  currents. 

then  falling   over   the   edge   of 

the  bank  into  the  bottom  of  the  pool.  As  the  sediment 
advances  by  successive  additions  to  its  steep  slope  in  front, 
it  gradually  fills  the  pool  up.  Its  progress  may  be  com- 
pared to  that  of  a  railway  embankment  formed  by  the  discharge 
of  waggon-loads  of  rubbish  down  its  end.  A  section  through  such 
an  embankment  would  reveal  a  series  of  bands  of  variously  col- 
oured materials  inclined  steeply  towards  the  direction  in  which 
the  waggon-loads  were  thrown  down.  Yet  the  top  of  the  em- 
bankment may  be  kept  quite  level  for  the  permanent  way.  The 
nearly  level  bands  (b,  c)  in  Fig.  12  represent  the  general  bottom 
on  which  the  sediment  accumulated,  while  the  steeper  lines  in 
the  lower  gravel  (a)  point  to  the  existence  and  direction  of  the 
currents  by  which  sediment  was  pushed  forward  along  that  bot- 
tom. 

As  the  river  flows  onward  through  a  gradually  expanding 
valley,  another  characteristic  feature  becomes  prominent.  Flank- 
ing each  side  of  the  flat  land  through  which  the  stream  pursues 
its  winding  course,  there  runs  a  steep  slope  or  bank  a  few  feet  or 


40  GEOLOGY 

yards  in  height,  terminating  above  in  a  second  or  higher  plain, 
which  again  may  be  bordered  with  another  similar  bank,  above 
which  there  may  lie  a  third  plain.  These  slopes  and  plains  form 
a  group  of  terraces,  rising  step  by  step  above  and  away  from  the 
river,  sometimes  to  a  height  of  several  hundred  feet,  and  occa- 
sionally to  the  number  of  6  or  8  or  even  more  (Fig.  13).  Here 
and  there,  by  the  narrowing  of  the  intervening  strip  of  plain, 
two  terraces  merge  into  one,  and  at  some  places  the  river  in 
winding  down  the  valley  has  cut  away  great  slices  from  the 
terraces,  perhaps  even  entirely  removing  them  and  eating  back 


Fig.   13. —  River-terraces. 

into  the  rock  out  of  which  the  valley  has  been  excavated.  Sec- 
tions are  thus  exposed  showing  a  succession  of  gravels,  sands,  and 
loams  like  those  of  the  present  river.  From  the  line  of  the  up- 
permost terrace  down  to  the  spits  of  shingle  now  forming  in  the 
channel,  we  have  evidently  a  chronologically  arranged  series  of 
river-deposits,  the  oldest  being  at  the  top  and  the  youngest  at 
the  bottom.  But  how  could  the  river  have  flowed  at  the  level  of 
these  high  gravels,  so  far  above  its  present  limits?  An  exam- 
ination of  the  behaviour  of  the  stream  during  floods  will  help 
towards  an  answer  to  this  questiton. 

When  from  heavy  rains  or  melted  snows  the  river  overflows 
its  banks,  it  spreads  out  over  the  level  ground  on  either  side. 
The  tract  liable  to  be  thus  submerged  during  inundations  is 
called  the  flood-plain.  As  the  river  rises  in  flood,  it  becomes 
more  and  more  turbid  from  the  quantity  of  mud  and  silt  poured 
into  it  by  its  tributaries  on  either  side.  Its  increase  in  volume 
likewise  augments  its  velocity,  and  consequently  its  power  of 


INFLUENCE  OF  RUNNING  WATER  41 

scouring  its  bed  and  of  transporting  the  coarser  detritus  resting 
there.  Large  quantites  of  shingle  >may  thus  be  swept  out  of  the 
ordinary  channel  and  be  strewn  across  the  nearer  parts  of  the 
flood-plain.  As  the  current  spreads  over  this  plain,  its  velocity 
and  transporting  capacity  diminish,  and  consequently  sediment 
begins  to  be  thrown  down.  Grass,  bushes,  and  trees,  standing 
on  the  flood-plain,  filter  some  of  the  sediment  out  of  the  water. 
Fine  mud  and  sand,  for  instance,  adhere  to  the  leaves  and  stems, 
whence  they  are  eventually  washed  off  by  rain  into  the  soil  under- 
neath. In  this  way,  the  flood-plain  is  gradually  heightened  by 
the  river  itself.  At  the  same  time,  the  bed  of  the  river  is  deep- 
ened by  the  scour  of  the  current,  until,  in  the  end,  even  the 
highest  floods  are  no  longer  able  to  inundate  the  flood-plain.  The 
difference  of  level  between  that  plain  and  the  surface  of  the 
river  gradually  increases;  by  degrees  the  river  begins  to  cut 
away  the  edges  of  the  terrace  which  it  cannot  now  overflow,  and 
to  form  a  new  flood-plain  at  a  lower  level.  In  this  manner,  it 
slowly  lowers  its  bed,  and  leaves  on  either  side  a  set  of  alluvial 
terraces  to  mark  successive  stages  in  the  process  of  excavation. 
If  during  this  process  the  level  of  the  land  should  be  raised,  the 
slope  of  the  rivers,  and  consequently  their  scour,  would  be  aug- 
mented, and  they  would  thereby  acquire  greater  capacity  for  the 
formation  of  terraces.  There  is  reason  to  believe  that  this  has 
taken  place  both  in  Europe  and  North  America. 

While  it  is  obvious  that  the  highest  terraces  must  be  the 
oldest,  and  that  the  series  is  progressively  younger  down  to  the 
terrace  that  is  being  formed  at  the  present  time,  nevertheless,  in 
the  materials  comprising  any  one  terrace,  those  lying  at  the  top 
must  be  the  youngest.  This  apparent  contradiction  arises  from 
the  double  action  of  the  river  in  eroding  its  bed  and  depositing  its 
sediment.  If  there  were  no  lowering  of  the  channel,  then  the 
deposits  would  follow  the  usual  order  of  sequence,  the  oldest  be- 
ing below  and  the  youngest  above.  This  order  is  maintained  in 
the  constituents  of  each  single  terrace,  for  the  lowermost  layers 
of  gravel  must  evidently  have  been  accumulated  before  the  de- 
posit of  those  that  overlie  them.  But  when  the  level  of  the  water 
is  lowered,  the  next  set  of  deposits  must,  though  younger,  lie 
at  a  lower  level  than  those  that  preceded  them.  In  no  case, 
however,  will  the  older  beds,  though  higher  in  position,  be  found 


42  GEOLOGY 

really  to  overlie  the  younger.  They  have  been  formed  at  differ- 
ent levels. 

The  gravel,  sand,  and  loam  laid  down  by  a  river  are  marked, 
as  we  have  seen,  by  an  arrangement  in  layers,  beds,  or  strata 
lying  one  upon  another.  This  stratified  disposition  indeed  is 
characteristic  of  all  sedimentary  accumulations,  and  is  best 
developed  where  currents  have  been  most  active  in  transporting 
and  assorting  the  various  materials.  It  is  the  feature  that  first 
catches  the  eye  in  any  river-bank,  where  a  section  of  the  older 
deposits  or  "  alluvium  "  is  exposed.  Beds  of  coarser  and  finer 
detritus  alternate  with  each  other,  but  the  coarsest  are  generally 
to  be  observed  below  and  the  finest  above.  The  "  deltas  "  accu- 
mulated by  rivers  in  lakes  and  in  the  sea  will  be  noticed  in 
Chapters  IV  and  VII. 

But  besides  the  inorganic  detritus  carried  forward  by  a  river, 
we  have  also  to  consider  the  fate  of  the  remains  of  plants  and 
the  carcases  of  animals  that  are  swept  down,  especially  during 
floods.  Swollen  by  sudden  and  heavy  rains,  a  river  will  rise 
above  its  ordinary  level  and  uproot  trees  and  shrubs.  On  sucli 
occasions,  too,  moles  and  rabbits  are  drowned  and  buried  in  their 
burrows  on  the  alluvial  flood-plain.  Birds,  insects,  and  even 
some  of  the  larger  mammals  are  from  time  to  time  drowned, 
swept  away  by  floods  and  buried  in  the  sediment,  and  their  re- 
mains, where  of  a  durable  kind  or  where  sufficiently  covered 
over,  may  be  preserved  for  an  indefinite  period.  The  shells  and 
fishes  living  in  the  river  itself  may  also  be  killed  during  the 
flood,  and  may  be  entombed  with  the  other  organisms  in  the 
sediment. 

SUMMARY. —  The  material  produced  by  the  universal  decay  of 
the  surface  of  the  land  is  washed  off  by  rain  and  swept  seawards 
by  brooks  and  rivers.  The  rate  at  which  the  general  level  of  the 
land  is  being  lowered  by  the  operation  of  running  water  may  be 
approximately  ascertained  by  measuring  or  estimating  the  amount 
of  mineral  matter  carried  seaward  every  year  from  a  definite 
region,  such  as  a  river-basin.  Taking  merely  the  matter  in 
mechanical  suspension,  and  assuming  that  the  proportion  of  it 
transported  annually  in  the  water  of  the  Mississippi  may  be  re- 
garded as  an  average  proportion  for  the  rivers  of  Europe,  we  find 
that  this  continent,  at  the  Mississippi  rate  of  degradation,  might 
be  reduced  to  the  sea-level  in  rather  less  than  6,000,000  years. 


INFLUENCE  OF  RUNNING  WATER  43 

In  pursuing  their  course  over  the  land,  running  waters  gradu- 
ally deepen  and  widen  the  channels  in  which  they  flow,  partly 
by  chemically  dissolving  the  rocks  and  partly  by.  rubbing  them 
down  by  the  friction  of.  the  transported  sand,  gravel,  and  stones. 
When  they  have  once  chosen  their  channels,  they  usually  keep  to 
them,  and  the  sinuous  windings,  at  first  determined  by  trifling 
inequalities  on  the  surface  of  country  across  which  the  streams 
began  to  flow,  are  gradually  deepened  into  picturesque  gorges. 
In  the  excavation  of  such  ravines,  waterfalls  play  an  important 
part  by  gradually  receding  up  stream.  River-channels,  espe- 
cially if  cut  deeply  into  the  solid  rock,  remain  as  enduring  monu- 
ments of  the  work  of  running  water. 

But  still  more  important  as  geological  records,  because  more 
frequent  and  covering  a  larger  area,  are  the  deposits  which  rivers 
leave  as  their  memorials.  Whatever  checks  the  velocity  of  a 
current  weakens  its  transporting  power,  and  causes  it  to  drop 
some  of  its  sediment  to  the  bottom.  Accordingly,  accumulations 
of  sediment  occur  at  the  foot  of  torrent  slopes,  along  the  lower 
and  more  level  ground,  especially  on  the  inner  or  concave  side  of 
the  loops,  over  the  flood-plains,  and  finally  in  the  deltas  formed 
where  rivers  enter  lakes  or  the  sea.  In  these  various  situations, 
thick  stratified  beds  of  silt,  sand,  and  gravel  may  be  formed, 
enclosing  the  remains  of  the  plants  and  animals  living  on  the 
land  at  the  time.  As  a  river  deepens  its  channel,  it  leaves  on 
either  side  alluvial  terraces  that  mark  successive  flood-plains 
over  which  it  has  flowed. 


44  GEOLOGY 


CHAPTER  IV. 

MEMORIALS  LEFT  BY  LAKES. 

FKESH-WATER  LAKES.—  According  to  the  law  stated  in 
last  chapter,  that  when  water  is  checked  in  its  flow,  it  must 
drop  some  of  its  sediment,  lakes  are  pre-eminently  places 
for  the  deposition  and  accumulation  of  mineral  matter.  In 
their  quiet  depths,  the  debris  worn  away  from  the  surface  of  the 
land  is  filtered  out  of  the  water  and  allowed  to  gather  undisturbed 
upon  the  bottom.  The  tributary  streams  may  enter  a  large  lake 
swollen  and  muddy,  but  the  escaping  river  is  transparent.  It 
is  evident,  therefore,  that  lakes  must  be  continually  silting  up, 
and  that  when  this  process  is  complete,  the  site  of  a  lake  will  be 
occupied  by  a  series  of  deposits  comprising  a  record  of  how  the 
water  was  made  to  disappear. 

To  those  who  know  the  aspect  of  lakes  only  in  fine  weather, 
they  may  seem  places  where  geological  operations  are  at  their 
very  minimum  of  activity.  The  placid  surface  of  the  water  rip- 
ples upon  beaches  of  gravel  or  spits  of  sand;  reeds  and  marshy 
plants  grow  out  into  the  shallows ;  the  few  streamlets  that  tumble 
down  from  the  surrounding  hills  furnish  perhaps  the  only  sounds 
that  break  the  stillness,  but  their  music  and  motion  are  at  once 
hushed  when  they  lose  themselves  in  the  lake.  The  scene  might 
serve  as  the  very  emblem  of  perfectly  undisturbed  conditions  of 
repose.  But  come  back  to  this  same  scene  during  an  autumn 
storm,  when  the  mists  have  gathered  all  round  the  hills,  and  the 
rain,  after  pouring  down  for  hours,  has  turned  every  gully  into 
the  track  of  a  roaring  torrent.  Each  tributary  brook,  hardly 
visible  perhaps  in  drought,  now  rushes  foaming  and  muddy  from 
its  dell  and  sweeps  out  into  the  lake.  The  large  streams  bear 
along  on  their  swift  brown  currents  trunks  of  trees,  leaves,  twigs, 
with  now  and  then  the  carcase  of  some  animal  that  has  been 
drowned  by  the  rising  flood.  Hour  after  hour,  from  every  side, 


MEMORIALS  LEFT  BY  LAKES  45 

these  innumerable  swollen  waters  bear  their  freights  of  gravel, 
sand,  and  mud  into  the  lake.  Hundreds  or  thousands  of  tons  of 
sediment  must  thus  be  swept  down  during  a  single  storm.  When 
we  multiply  this  result  by  the  number  of  storms  in  a  year  and 
by  the  number  of  years  in  an  ordinary  human  life,  we  need  not 
be  surprised  to  be  told  that  even  within  the  memory  of  the  pres- 
ent generation,  and  still  more  within  historic  times,  conspicuous 
changes  have  taken  place  in  many  lakes. 

Filling  up  of  Lakes. —  In  the  Lake  of  Lucerne,  for  exam- 
ple, the  River  Reuss,  which  bears  down  the  drainage  of  the  huge 
mountains  round  the  St.  Gothard,  deposits  about  7,000,000  cubic 
feet  of  sediment  every  year.  Since  the  year  1714  the  Kander, 


Fig.  14. —  Alluvial  terraces  on  the  side  of  an  emptied  reservoir. 

which  drains  the  northern  flanks  of  the  centre  of  the  Bernese 
Oberland,  is  said  to  have  thrown  into  the  lower  end  of  the  Lake 
of  Thun  such  an  amount  of  sediment  as  to  form  an  area  of  230 
acres,  now  partly  woodland,  partly  meadow  and  marsh.  Since 
the  time  of  the  Romans,  the  Rhone  has  filled  up  the  upper  end 
of  the  Lake  of  Geneva  to  such  an  extent  that  a  Roman  harbour, 
still  called  Port  Valais,  is  now  nearly  two  miles  from  the  edge  of 
the  lake,  the  intervening  ground  having  been  converted  first  into 
marshes  and  then  into  meadows  and  farms. 

It  is  at  the  mouths  of  streams  pouring  into  a  lake  that  the 
process  of  filling  up  is  most  rapid  and  striking.  But  it  may  be 
detected  at  many  other  places  round  the  margin.  Instructive 
lessons  on  this  subject  may  be  learned  at  a  reservoir  formed  by 
damming  back  the  waters  of  a  steep-sided  valley,  and  liable  to 
be  sometimes  nearly  dry  (Fig.  14).  In  such  a  situation,  when 
the  water  is  low,  it  may  be  noticed  that  a  series  of  parallel  lines 
runs  all  round  the  sides  of  the  reservoir,  and  that  these  lines 
consist  of  gravel,  sand,  or  earth.  Each  of  them  marks  a  former 


46  GEOLOGY 

level  of  the  water,  and  they  show  that  the  reservoir  was  not 
drained  off  at  once  but  intermittently,  each  pause  in  the  diminu- 
tion of  level  being  marked  by  a  line  of  sediment.  It  is  easy  to 
watch  how  these  lines  are  formed  along  the  present  margin  of 
the  water.  The  loose  debris  from  the  bare  slope  above,  partly  by 
its  own  gravitation,  partly  by  the  wash  of  rain,  slides  down  into 
the  water.  But  as  soon  as  it  gets  there,  its  further  downward 
movement  is  arrested.  By  the  ripple  of  the  water  it  is  gently 
moved  up  and  down,  but  keeps  on  the  whole  just  below  the  line 
to  which  the  water  reaches.  So  long  as  it  is  concealed  under  the 
water,  its  position  and  extent  can  hardly  be  realised.  But  as 


Fig.  15. —  Parallel  roads  of  Glen  Roy. 

soon  as  the  level  of  the  reservoir  sinks,  the  sediment  is  left  as  a 
marked  shelf  or  terrace.  In  natural  lakes,  the  same  process  is 
going  on,  though  hardly  recognisable,  because  concealed  under 
the  water.  But  if  by  any  means  a  lake  could  be  rapidly  emptied, 
its  former  level  would  be  marked  by  a  shelf  or  alluvial  terrace. 
In  some  cases,  the  barrier  of  a  lake  has  been  removed,  and  the 
sinking  of  the  water  has  revealed  the  terrace.  The  famous 
"  parallel  roads  "  of  Glen  Roy,  in  the  west  of  Scotland,  are  nota- 
ble examples  (Fig.  15) .  The  valleys  in  that  region  were  ancient- 
ly dammed  up  by  large  glaciers.  The  drainage  accumulated  be- 
hind the  ice,  filled  up  the  valleys  and  converted  them  into  a  series 


MEMORIALS  LEFT  BY  LAKES  47 

of  lakes  or  fresh-water  "  fjords/'  The  former  levels  of  these 
sheets  of  water  and  the  successive  stages  of  their  diminution  and 
disappearance  are  shown  by  the  series  of  alluvial  shelves  known 
as  "  parallel  roads."  The  highest  of  these  is  1,140  feet,  the  mid- 
dle 1,059  feet,  and  the  lowest  847  feet  above  the  level  of  the  sea. 
Thus,  partly  by  the  washing  of  detritus  down  from  the  ad- 
joining slopes  by  rain,  partly  by  the  sediment  carried  into  them 
by  streams,  and  partly  by  the  growth  of  marshy  vegetation  along 
their  margins,  lakes  are  visibly  diminishing  in  size.  In  moun- 
tainous countries,  every  stage  of  this  appearance  may  be  ob- 
served (Fig.  16).  Where  the  lakes  are  deep,  the  tongues  of 


are 


A  B 

Fig.  16. —  Stages  in  the  filling  up  of  a  lake.  In  A  two  streamlets  „.„ 
represented  as  pouring  their  "  deltas  "  into  a  lake.  In  B  they  have 
filled  the  lake  up,  converting  it  into  a  meadow  across  which  they  wind 
on  their  way  down  the  valley. 

sediment  or  "  deltas  "  which  the  streams  push  in  front  of  them 
have  not  yet  been  able  to  advance  far  from  the  shore.  In  other 
cases,  every  tributary  has  built  up  an  alluvial  plain  which  grows 
outwards  and  along  the  coast,  until  it  unites  with  those  of  its 
neighbours  to  form  a  nearly  continuous  belt  of  flat  meadow  and 
marsh  round  the  lake.  By  degrees,  as  this  belt  increases  in 
width,  the  lake  narrows,  until  the  whole  tract  is  finally  converted 
into  an  alluvial  plain,  through  which  the  river  and  its  tribu- 
taries wind  on  their  way  to  lower  levels.  The  successive  flat 
meadow-like  expansions  of  valleys  among  hills  and  mountains 
were  probably  in  most  cases  originally  lakes  which  have  in  this 
manner  been  gradually  filled  up. 

Lake  Deposits. — The  bottoms  of  lakes  must  evidently  con- 
tain many  interesting  relics.  Dispersed  through  the  shingle, 
sand,  and  mud  that  gather  there,  are  the  remains  of  plants  and 
animals  that  lived  on  the  surrounding  land.  Leaves,  fruits, 
twigs,  branches,  and  trunks  embedded  in  the  silt  may  preserve 
for  an  indefinite  period  their  record  of  the  vegetation  of  the  time. 


48  GEOLOGY 

The  wings  or  wing-cases  of  insects,  the  shells  of  land-snails,  the 
bones  of  birds  and  mammals,  carried  down  into  the  depths  of  a 
lake  and  entombed  in  the  silt  there,  will  remain  as  a  chronicle 
of  the  kind  of  animals  that  haunted  the  surrounding  hills  and 
valleys. 

The  layers  of  gravel,  sand,  and  silt  laid  down  on  the  floor  of  a 
lake  differ  in  some  respects  from  those  deposited  in  the  terraces 
of  a  river,  being  generally  finer  in  grain,  and  including  a  larger 
proportion  of  silt,  mud,  or  clay  among  them,  especially  away 
from  the  margin  of  the  lake.  They  are,  no  doubt,  further  dis- 
tinguished by  the  greater  abundance  of  the  remains  of  plants  and 
animals  preserved  in  them. 

But  lakes  likewise  serve  as  receptacles  for  a  series  of  deposits 
which  are  peculiar  to  them,  and  which  consequently  have  much 
interest  and  importance  as  they  furnish  a  ready  means  of  detect- 
ing the  sites  of  lakes  that  have  long  disappeared.  The  molluscs 
that  live  in  lacustrine  waters  are  distinct  from  the  snails  of  the 
adjoining  shores.  Their  dead  shells  gather  on  the  bottoms  of 
some  lakes  in  such  numbers  as  to  form  there  a  deposit  of  the 
white  crumbling  marl,  already  referred  to.  In  the  course  of 
time  this  deposit  may  grow  to  be  many  feet  or  yards  in  thick- 
ness. The  shells  in  the  upper  parts  may  be  quite  fresh,  some 
of  the  animals  having  only  recently  died ;  but  they  become  more 
and  more  decayed  below  until,  towards  the  bottom  of  the  deposit, 
the  marl  passes  into  a  more  compact  chalk-like  substance  in  which 

few  or  no  shells  may 
be  recognisable  (Fig.  17). 
On  the  sites  of  lakes 
that  have  been  naturally 
filled  up  or  artificially 
drained,  such  marl  has 
been  extensively  dug  as 
a  manure  for  land.  Be- 
sides the  shells  from  the 
decay  of  which  it  is 
Fig.  17.— Piece  of  shell-marl  containing  chiefly  formed,  it  some- 
shells  of  Ltmnwa  peregra.  .  .11  , 

times  yields  the  bones  of 

deer,  oxen,  and  other  animals,  whose  carcases  must  originally 
have  sunk  to  the  bottom  of  the  lake  and  been  there  gradually 
covered  up  in  the  growing  mass  of  marl.  Many  examples  of 


MEMORIALS  LEFT  BY  LAKES  49 

,    » 

these  marl-deposits  are  to  be  found  among  the  drained  lakes  of 
Scotland  and  Ireland. 

Yet  another  peculiar  accumulation  is  met  with  on  the  bottom 
of  some  lakes,  particularly  in  Sweden.  In  the  neighbourhood 
of  banks  of  reeds  and  on  the  sloping  shallows  of  the  larger  lakes, 
a  deposit  of  hydrated  peroxide  of  iron  takes  place,  in  the  form 
of  concretions  varying  in  size  from  small  grains  like  gunpowder 
up  to  cakes  measuring  six  inches  across.  The  iron  is  no  doubt 
dissolved  out  of  the  rocks  of  the  neighbourhood  by  water  con- 
taining organic  acids  or  carbonic  acid.  In  this  condition,  it  is 
liable  to  be  oxidised  on  exposure.  As  after  oxidation  it  can  no 
longer  be  retained  in  solution,  it  is  precipitated  to  the  bottom 
where  it  collects  in  grains  which  by  successive  additions  to  their 
surface  become  pellets,  balls,  or  cakes.  Possibly  some  of  the 
microscopic  plants  (diatoms)  which  abound  on  the  bottoms  of 
the  lakes  may  facilitate  the  accumulation  of  the  iron  by  abstract- 
ing this  substance  from  the  water  and  depositing  it  inside  their 
siliceous  coverings.  Beds  of  concretionary  brown  ironstone  are 
formed  in  Sweden  from  10  to  200  yards  long,  5  to  15  yards 
broad,  and  from  8  to  30  inches  thick.  During  winter  when 
the  lakes  are  frozen  over,  the  iron  is  raked  up  from  the  bottom 
through  holes  made  for  the  purpose  in  the  ice,  and  is  largely 
used  for  the  manufacture  of  iron  in  the  Swedish  furnaces. 
When  the  iron  has  been  removed,  it  begins  to  form  again,  and 
instances  are  known  where,  after  the  supply  had  been  completely 
exhausted,  beds  several  inches  in  thickness  were  formed  again  in 
twenty-six  years. 

SALT-LAKES. —  The  salt-lakes  of  desert  regions  present  a 
wholly  peculiar  series  of  deposits.  These  sheets  of  water  have  no 
outlet ;  yet  there  is  reason  to  believe  that  most  of  them  were  at 
first  fresh,  and  discharged  their  outflow  like  ordinary  lakes. 
Owing  to  geological  changes  of  level  and  of  climate,  they  have 
long  ceased  to  overflow.  The  water  that  runs  into  them,  instead 
of  escaping  by  a  river,  is  evaporated  back  into  the  air.  But  the 
various  mineral  salts  carried  by  it  in  solution  from  rocks  and 
soils  are  not  evaporated  also.  They  remain  behind  in  the  lakes, 
which  are  consequently  becoming  gradually  salter.  Among  the 
salts  thus  introduced,  common  salt  (sodium-chloride)  and  gyp- 
sum (calcium-sulphate)  are  two  of  the  most  important.  These 
substances,  as  the  water  evaporates  in  the  shallows,  bays,  and 


50  GEOLOGY 

pools,  are  precipitated  to  the  bottom  where  they  form  solid 
layers  of  salt  and  gypsum.  The  latter  substance  begins  to  be 
thrown  down  when  37  per  cent  of  the  water  containing  it  has 
been  evaporated.  The  sodium-chloride  does  not  appear  until  93 
per  cent  of  the  water  has  disappeared.  In  the  order  of  deposit, 
therefore,  gypsum  comes  before  the  salt  (see  chap.  10).  Some 
bitter  lakes  contain  sodium-carbonate,  in  others  magnesium- 
chloride  is  abundant.  The  Dead  Sea,  the  Great  Salt  Lake  of 
Utah,  and  many  other  salt  lakes  and  inland  seas  furnish  inter- 
esting evidence  of  the  way  in  which  they  have  gradually  changed. 
In  the  upper  terraces  of  the  Great  Salt  Lake,  1,000  feet  or  more 
above  the  present  level  of  the  water,  fresh-water  shells  occur, 
showing  that  the  basin  was  at  first  fresh.  The  valley-bottoms 
around  saline  lakes  are  now  crusted  with  gypsum,  salt,  or  other 
efflorescence,  and  their  waters  are  almost  wholly  devoid  of  life. 
Such  conditions  as  these  help  us  to  understand  how  great  de- 
posits of  gypsum  and  rock-salt  were  formed  in  England,  Ger- 
many, and  many  other  regions  where  the  climate  would  not  now 
permit  of  any  such  condensation  of  the  water  (Chapter  XXII). 

SUMMARY. —  The  records  inscribed  by  lakes  in  geological  his- 
tory consist  of  layers  of  various  kinds  of  sediment.  These  de- 
posits may  form  mere  shelves  or  terraces  along  the  margin  of 
the  water  which,  if  drained  off,  will  leave  them  as  evidence  of  its 
former  levels.  By  the  long-continued  operations  of  rain,  brooks, 
and  rivers,  continually  bringing  down  sediment,  lakes  are  gradu- 
ally filled  up  with  alluvium,  and  finally  become  flat  meadow-land 
with  tributary  streams  winding  through  it.  The  deposits  that 
thus  replace  the  lacustrine  water  consist  mainly  of  sand  or  gravel 
near  shore,  while  finer  silt  occupies  the  site  of  the  deeper  water. 
They  may  also  include  beds  of  marl  formed  of  fresh-water  shells, 
and  sheets  of  brown  iron  ore.  Throughout  them  all,  remains  of 
the  plants  and  animals  of  the  surrounding  land  are  likely  to  be 
entombed  and  preserved. 

Salt  lakes  leave,  as  their  enduring  memorial,  beds  of  rock-salt 
and  gypsum,  sometimes  carbonate  of  soda  and  other  salts.  Many 
of  them  were  at  first  fresh,  as  is  shown  by  the  presence  of  ordi- 
nary fresh-water  shells  in  their  upper  terraces.  But  by  change 
of  climate  and  long-continued  excess  of  evaporation  over  precipi- 
tation, the  water  has  gradually  become  more  and  more  saline,  and 
has  sometimes  disappeared  altogether,  leaving  behind  it  de- 
posits of  common  salt,  gypsum,  and  other  chemical  precipitates. 


HOW  SPRINGS  LEAVE  THEIR  MARK  51 


CHAPTER  V. 

HOW  SPRINGS  LEAVE  THEIR  MARK  IN  GEOLOGICAL  HISTORY. 

THE  changes  made  by  running  water  upon  the  land  are  not 
confined  to  that  portion  of  the  rainfall  which  courses 
along  the  surface.  Even  when  it  sinks  underground  and 
seems  to  have  passed  out  of  the  general  circulation,  the  subter- 
ranean moisture  does  not  remain  inactive.  After  travelling  for 
a  longer  or  shorter  distance  through  the  pores  of  rocks,  or  along 
their  joints  and  other  divisional  planes,  it  finds  its  way  once  more 
to  daylight  and  reappears  in  Springs.  In  this  underground 
journey,  it  corrodes  rocks,  somewhat  in  the  same  way  as  rain  at- 
tacks those  that  are  exposed  to  the  outer  air,  and  it  works  some 
curious  changes  upon  the  face  of  the  land.  Subterranean  water 
thus  leaves  distinct  and  characteristic  memorials  as  its  contribu- 
tion to  geological  history. 

There  are  two  aspects  in  which  the  work  of  underground  water 
may  be  considered  here.  In  the  first  place,  portions  of  the  sub- 
stance of  subterranean  rocks  are  removed  by  the  percolating  water 
and  in  large  measure  carried  up  above  ground;  in  the  second 
place,  some  of  these  materials  are  laid  down  again  in  a  new  form 
and  take  a  conspicuous  place  among  the  geological  monuments  of 
their  time. 

In  the  removal  of  mineral  substance,  water  percolating  through 
rocks  acts  in  two  distinct  ways,  mechanical  and  chemical,  each  of 
which  shows  itself  in  its  own  peculiar  effects  upon  the  surface. 

(1)  MECHANICAL  ACTION. —  While  slowly  filtering  through 
porous  materials,  water  tends  to  remove  loose  particles  and  thus 
to  lessen  the  support  of  overlying  rocks.  But  even  where  there 
is  no  transport,  the  water  itself,  by  saturating  a  porous  layer  that 
rests  upon  a  more  or  less  impervious  one,  loosens  the  cohesion  of 
that  porous  layer.  The  overlying  mass  of  rock  is  thus  made  to 
rest  upon  a  watery  and  weakened  platform,  and  if  from  its  posi- 


52  GEOLOGY 

tion  it  should  have  a  tendency  to  gravitate  in  any  given  direction, 
it  may  at  last  yield  to  this  tendency  and  slide  downwards.  Along 
the  sides  of  sea-cliffs,  on  the  precipitous  slopes  of  valleys  or  river- 
gorges,  or  on  the  declivities  of  hills  and  mountains,  the  conditions 
are  often  extremely  favourable  for  the  descent  of  large  masses 
of  rock  from  higher  to  lower  levels. 

Remarkable  illustrations  of  such  Landslips,  as  they  are  called, 
have  been  observed  along  the  south  coast  of  England,  where  cer- 
tain porous  sandy  rocks  underlying  a  thick  sheet  of  chalk  rest 
upon  more  or  less  impervious  clays,  which,  by  arresting  the 
water  in  its  descent,  throw  it  out  along  the  base  of  the  slopes. 
After  much  wet  weather,  the  upper  surface  of  these  clays  be- 
comes, as  it  were,  lubricated  by  the  accumulation  of  water,  and 
large  slices  of  the  overlying  rocks,  having  their  support  thereby 
weakened,  break  off  from  the  solid  cliffs  behind  and  slide  down 
towards  the  sea.  The  most  memorable  example  occurred  at 
Christmas  time,  in  the  year  1839,  on  the  coast  of  Devonshire  not 
far  from  Axmouth.  At  that  locality,  the  chalk-downs  end  off  in 
a  line  of  broken  cliff  some  500  feet  above  the  sea.  From  the 
edge  of  the  downs  flanked  by  this  cliff  a  tract  about  800  yards 
long,  containing  not  less  than  30  acres  of  arable  land,  sank  down 
with  all  its  fields,  hedgerows,  and  pathways.  This  sunken  mass, 
where  it  broke  away  from  the  upland,  left  behind  it  a  new  cliff, 
showing  along  the  crest  the  truncated  ends  of  the  fields,  of  which 
the  continuation  was  to  be  found  in  a  chasm  more  than  200  feet 
deep.  While  the  ground  sank  into  this  defile  and  was  tilted 
steeply  towards  the  base  of  the  cliff,  it  was  torn  up  by  a  long 
rent  running  on  the  whole  in  the  line  of  the  cliff,  and  by  many 
parallel  and  transverse  fissures.  Half  a  century  has  passed  away 
since  this  landslip  occurred.  The  cliff  remains  much  as  it  was  at 
first,  and  the  sunken  fields  with  their  bits  of  hedgerow  still 
slope  steeply  down  to  the  bottom  of  the  declivity  (Fig.  18) .  But 
the  lapse  of  time  has  allowed  the  influence  of  the  atmosphere  to 
come  into  play.  The  outstanding  dislocated  fragments  with 
their  vertical  walls  and  flat  tops,  showing  segments  of  fields,  have 
been  gradually  worn  into  tower-like  masses  with  sloping  de- 
clivities of  debris.  The  long  parallel  rent  has  been  widened  by 
rain  into  a  defile  with  shelving  sides.  Everywhere  the  rawness 
of  the  original  fissures  has  been  softened  by  the  rich  tapestry  of 
verdure  which  the  genial  climate  of  that  southern  coast  fosters 


HOW  SPRINGS  LEAVE  THEIR  MARK 


53 


54  GEOLOGY 

in  every  sheltered  nook.  But  the  scars  have  not  been  healed, 
and  they  will  no  doubt  remain  still  visible  for  many  a  year  to 
come. 

Along  the  south  coast  of  England,  many  landslips,  of  which 
there  is  no  historical  record,  have  produced  some  of  the  most 
picturesque  scenery  of  that  region.  Masses  that  have  slipped 
away  from  the  main  cliff  have  so  grouped  themselves  down  the 
slopes  that  hillocks  and  hollows  succeed  each  other  in  endless 
confusion,  as  in  the  well-known  Undercliff  of  the  Isle  of  Wight. 
Some  of  the  tumbled  rocks  are  still  fresh  enough  to  show  that 
they  have  fallen  at  no  very  remote  period,  or  even  that  the  slip- 
ping still  continues;  others,  again,  have  yielded  so  much  to  the 
weather  that  their  date  doubtless  goes  far  back  into  the  past,  and 
some  of  them  are  crowned  with  what  are  now  venerable  ruins. 

The  most  stupendous  landslips  on  record  have  occurred  in 
mountainous  countries.  Upwards  of  150  destructive  examples 
have  been  chronicled  in  Switzerland.  Of  these,  one  of  the  most 
memorable  was  that  of  the  Kossberg,  a  mountain  lying  behind  the 
Eigi,  and  composed  of  thick  masses  of  hard  red  sandstone  and 
conglomerate  so  arranged  as  to  slope  down  into  the  valley  of 
Goldau.  The  summer  of  the  year  1806  having  been  particularly 
wet,  so  large  an  amount  of  water  had  collected  in  the  more  porous 
layers  of  rock  as  to  weaken  the  support  of  the  overlying  mass ; 
consequently  a  large  part  of  the  side  of  the  mountain  suddenly 
gave  way  and  rushed  down  into  the  valley,  burying  under  the 
debris  about  a  square  German  mile  of  fertile  land,  four  villages 
containing  330  cottages  and  outhouses,  and  457  inhabitants.  To 
this  day,  huge  angular  blocks  of  sandstone  lying  on  the  farther 
side  of  the  valley  bear  witness  to  the  destruction  caused  by  this 
landslip,  and  the  scar  on  the  mountain-slope  whence  the  fallen 
masses  descended  is  still  fresh. 

(2)  CHEMICAL  ACTION — (a)  Solution. —  But  it  is  by  its 
chemical  action  on  the  rocks  through  which  it  flows  that  sub- 
terranean water  removes  by  far  the  largest  amount  of  mineral 
matter,  and  produces  the  greatest  geological  change.  Even  pure 
water  will  dissolve  a  minute  quantity  of  the  substance  of  many 
rocks.  But  rain  is  far  from  being  chemically  pure  water.  In 
previous  chapters  it  has  been  described  as  taking  oxygen  and  car- 
bonic acid  out  of  the  air  in  its  descent,  and  abstracting  organic 
acids  and  carbonic  acid  from  the  soil  through  which  it  sinks. 


HOW  SPRINGS  LEAVE  THEIR  MARK  55 

By  help  of  these  Ingredients,  it  is  enabled  to  attack  even  the  most 
durable  rocks,  and  to  carry  some  of  their  dissolved  substance  up 
to  the  surface  of  the  ground. 

One  of  the  substances  most  readily  attacked  and  removed  even 
by  pure  water  is  the  mineral  known  as  carbonate  of  lime.  Among 
other  impurities,  natural  waters  generally  contain  carbonic  acid, 
which  may  be  derived  from  the  air  or  from  the  soil ;  occasionally 
from  some  deeper  subterranean  source.  The  presence  of  this  acid 
gives  the  water  greatly  increased  solvent  power,  enabling  it 
readily  to  attack  carbonate  of  lime,  whether  in  the  form  of  lime- 
stone, or  diffused  through  rocks  composed  mainly  of  other  sub- 
stances. Even  lime,  which  is  not  in  the  form  of  carbonate,  but 
is  united  with  silica  in  various  crystalline  minerals  (silicates,  Ch. 
X),  may  by  this  means  be  decomposed  and  combined  with  car- 
bonic acid.  It  is  then  removed  in  solution  as  carbonate.  So 
long  as  the  water  retains  enough  of  free  carbonic  acid,  it  can 
keep  the  carbonate  of  lime  in  solution  and  carry  it  onward. 

Limestone  is  a  rock  almost  entirely  composed  of  carbonate  of 
lime.  It  occurs  in  most  parts  of  the  world,  covering  sometimes 
tracts  of  hundreds  or  thousands  of  square  miles,  and  often  rising 
into  groups  of  hills,  or  even  into  ranges  of  mountains  (see  Lime- 
stone, Ch.  XI).  The  abundance  of  this  rock  affords  ample  oppor- 
tunity for  the  display  of  the  solvent  action  of  subterranean 
water.  Trickling  down  the  vertical  joints  and  along  the  planes 
between  the  limestone  beds,  the  water  dissolves  and  removes  the 
stone,  until  in  the  course  of  centuries  these  passages  are  gradu- 
ally enlarged  into  clefts,  tunnels,  and  caverns.  The  ground  be- 
comes honey-combed  with  openings  into  dark  subterranean- 
chambers,  and  running  streams  fall  into  these  openings  and  con- 
tinue their  course  underground. 

Every  country  which  possesses  large  limestone  tracts  furnishes 
examples  of  the  way  in  which  such  labyrinthine  tunnels  and 
systems  of  caverns  are  excavated.  In  England,  for  example,  the 
Peak  Cavern  of  Derbyshire  is  believed  to  be  2,300  feet  long,  and 
in  some  places  120  feet  high.  On  a  much  more  magnificent 
scale  are  the  caverns  of  Adelsberg  near  Trieste,  which  have  been 
explored  to  a  distance  of  between  four  and  five  miles,  but  are 
probably  still  more  extensive.  The  river  Poik  has  broken  into 
one  part  of  the  labyrinth  of  chambers,  through  which  it  rushes 
before  emerging  again  to  the  light.  Narrow  tunnels  expand  into 


56  GEOLOGY 

spacious  halls,  beyond  which  egress  is  again  afforded  by  low 
passages  into  other  lofty  recesses.  The  most  stupendous  cham- 
ber measures  669  feet  in  length,  630  feet  in  breadth,  and  111  feet 
in  height.  From  the  roofs  hang  pendent  white  stalactites, 
which,  in  uniting  with  the  floor,  form  pillars  of  endless  vari- 
eties of  form  and  size.  Still  more  gigantic  is  the  system  of  sub- 
terranean passages  in  the  Mammoth  Cave  of  Kentucky,  the  acces- 
sible parts  of  which  are  believed  to  have  a  combined  length  of 
about  150  miles.  The  largest  cavern  in  this  vast  labyrinth  has 
an.  area  of  two  acres,  and  is  covered  by  a  vault  125  feet  high. 

Of  the  mineral  matter  dissolved  by  permeating  water  out  of 
the  rocks  underground,  by  far  the  larger  part  is  discharged  by 
springs  into  rivers,  and  ultimately  finds  its  way  to  the  sea.  The 
total  amount  of  material  thus  supplied  to  the  sea  every  year 
must  be  enormous.  Much  of  it,  indeed,  is  abstracted  from  ocean- 


Fig.  19. —  Section  of  cavern  with  stalactites  and  stalagmite. 

water  by  the  numerous  tribes  of  marine  plants  and  animals.  In 
particular,  the  lime,  silica,  and  organic  matter  are  readily  seized 
upon  to  build  up  the  framework  and  furnish  the  food  of  these 
creatures.  But  probably  more  mineral  matter  is  supplied  in  solu- 
tion than  is  required  by  the  organisms  of  the  sea,  in  which  case 
the  water  of  the  sea  must  be  gradually  growing  heavier  and  salter. 
(5)  Deposition. —  But  it  is  the  smaller  proportion  of  the 
material  not  conveyed  into  the  sea  that  specially  demands  atten- 
tion. Every  spring,  even  the  purest  and  most  transparent,  con- 
tains mineral  solutions  in  sufficient  quantity  to  be  detected  by 
chemical  analysis.  Hence  all  plants  and  animals  that  drink  the 
water  of  springs  and  rivers  necessarily  imbibe  these  solutions 
which,  indeed,  supply  some  of  the  mineral  salts  whereof  the 


HOW  SPRINGS  LEAVE  THEIR  MARK  57 

harder  parts  both  of  plants  and  animals  are  constructed.  Many 
springs,  however,  contain  so  large  a  proportion  of  mineral  matter, 
that  when  they  reach  the  surface  and  begin  to  evaporate^  they 
drop  their  solutions  as  a  precipitate,  which  settles  down  upon 
the  bottom  or  on  objects  within  reach  of  the  water.  After  years 
of  undisturbed  continuance,  extensive  sheets  of  mineral  material 
may  in  this  manner  be  accumulated,  which  remain  as  enduring 
monuments  of  the  work  of  underground  water,  even  long  after 
the  springs  that  formed  them  have  ceased  to  flow. 

Calcareous  Springs. —  Among  the  accumulations  of  this  nature 
by  far  the  most  frequent  and  important  are  those  formed  by  what 
are  called  Calcareous  Springs.  In  regions  abounding  in  lime- 
stone or  rocks  containing  much  carbonate  of  lime,  the  subter- 
ranean waters  which,  as  we  have  seen,  gradually  erode  such  vast 
systems  of  tunnels,  clefts,  and  caverns,  carry 
away  the  dissolved  rock,  and  retain  it  in  solu- 
tion only  so  long  as  they  can  keep  their  car- 
bonic acid.  As  soon  as  they  begin  to  evapo- 
rate and  to  lose  some  of  this  acid,  they  lose 
also  the  power  of  retaining  so  much  carbonate 
of  lime  in  solution.  This  substance  is  accord- 
ingly dropped  as  a  fine  white  precipitate, 
which  gathers  on  the  surfaces  over  which  the 
water  trickles  or  flows. 

The  most  familiar  example  of  this  process 
is  to  be  seen  under  the  arches  of  bridges  and 
vaults.  Long  pendent  white  stalks  or  stalac- 
tites hang  from  between  the  joints  of  the 
masonry,  while  wavy  ribs  of  the  same  sub- 
stance run  down  the  piers  or  walls,  and  even 
collect  upon  the  ground  (stalagmite') .  A  few 
years  may  suffice  to  drape  an  archway  with  a 
kind  of  fringe  of  these  pencil-like  icicles  of 
stone.  Percolating  from  above  through  the 
joints  between  the  stones  of  the  masonry,  the  Fig.  20. —  Section 
rain-water,  armed  with  its  minute  proportion  |frjins  *«cces- 
of  carbonic  acid  at  once  attacks  the  lime  of  growth  in  a  stal- 
the  mortar  and  forms  carbonate  of  lime,  which  actlte- 
is  carried  downward  in  solution.  Arriving  at  the  surface  of  the 
arch,  the  water  gathers  into  a  drop,  which  remains  hanging  there 


58  GEOLOGY 

for  a  brief  interval  before  it  falls  to  the  ground.  That  interval 
suffices  to  allow  some  of  the  carbonic  acid  to  escape,  and  some  of 
the  water  to  evaporate.  Consequently,  round  the  outer  rim  of 
the  drop  a  slight  precipitation  of  white  chalky  carbonate  of  lime 
takes  place.  This  circular  pellicle,  after  the  drop  falls,  is  in- 
creased by  a  similar  deposit  from  the  next  drop,  and  thus  drop  by 
drop  the  original  rim  or  ring  is  gradually  lengthened  into  a  tube 
which  may  eventually  be  filled  up  inside,  and  may  be  thickened 
irregularly  outside  by  the  trickle  of  calcareous  water  (Fig.  20). 
But  the  deposition  on  the.  roof  does  not  exhaust  the  stock  of 
dissolved  carbonate.  When  the  drops  reach  the  ground  the  same 
process  of  evaporation  and  precipitation  continues.  Little 
mounds  of  the  same  white  substance  are  built  up  on  the  floor, 
and,  if  the  place  remain  undisturbed,  may  grow  until  they  unite 
with  the  stalactites  from  the  roof,  forming  white  pillars  that 
reach  from  floor  to  ceiling  (Fig.  19). 

It  is  in  limestone  caverns  that  stalactitic  growth  is  seen  on  the 
most  colossal  scale.  These  quiet  recesses  having  remained  un- 
disturbed for  many  ages,  the  process  of  solution  and  precipitation 
has  advanced  without  interruption  until,  in  many  cases,  vast 
caverns  have  been  transformed  into  grottoes  of  the  most  marvel- 
lous beauty.  While  glistening  fringes  and  curtains  of  crystalline 
carbonate  of  lime,  or  spar,  as  it  is  popularly  called,  hang  in  end- 
less variety  and  beauty  of  form  from  the  roof.  Pillars  of  every 
dimension,  from  slender  wands  up  to  thick-ribbed  columns  like 
those  of  a  cathedral,  connect  the  roof  and  the  pavement.  The 
walls,  projecting  in  massive  buttresses  and  retiring  into  alcoves, 
are  everywhere  festooned  with  a  grotesque  drapery  of  stone.  The 
floor  is  crowded  with  mounds  and  bosses  of  strangely  imitative 
forms  which  recall  some  of  the  oddest  shapes  above  ground. 
Wandering  through  such  a  scene,  the  visitor  somehow  feels  him- 
self to  be  in  another  world,  where  much  of  the  architecture  and 
ornament  belongs  to  styles  utterly  unlike  those  which  can  be 
seen  anywhere  else. 

The  material  composing  stalactite  and  stalagmite  is  at  first, 
as  already  stated,  a  fine  white  chalky  pulp-like  substance  which 
dries  into  a  white  powder.  But  as  the  deposition  continues,  the 
older  layers,  being  impregnated  with  calcareous  water,  receive  a 
precipitation  of  carbonate  of  lime  between  their  minute  pores 
and  crevices,  and  assume  a  crystalline  structure.  Solidifying 


HOW  SPRINGS  LEAVE  THEIR  MARK  59 

and  hardening  by  degrees,  they  end  by  becoming  a  compact  crys- 
talline stone  (spar)  which  rings  under  the  hammer. 

The  numerous  caverns  of  limestone  districts  have  offered  ready 
shelter  to  various  kinds  of  wild  animals  and  to  man  himself. 
Some  of  them  (Bone-Caves)  have  been  hyaena-dens,  and  from 
under  their  hard  floor  of  stalagmite  the  bones  of  hyaenas  and 
of  the  creatures  they  fed  upon  are  disinterred  in  abundance. 
Eude  human  implements  have  likewise  been  obtained  from  the 
same  deposits,  showing  that  man  was  contemporary  with  animals 
which  have  long  been  extinct.  The  solvent  action  of  under- 
ground water  has  thus  been  of  the  utmost  service  in  geological 
history,  first,  in  forming  caverns  that  could  be  used  as  retreats, 
and  then  in  providing  a  hard  incrustation  which  should  effectu- 
ally seal  up  and  preserve  the  relics  of  the  denizens  left  upon  the 
cavern-floors. 

Calcareous  springs,  issuing  from  limestone  or  other  rock 
abounding  in  lime,  deposit  carbonate  of  lime  as  a  white  pre- 
cipitate. So  large  is  the  proportion  of  mineral  contained  by 
some  waters  that  thick  and  extensive  accumulations  of  it  have 
been  formed.  The  substance  thus  deposited  is  known  by  the 
name  of  Calcareous  Tufa,,  Calc-sinter,  or  Travertine.  It  varies 
in  texture,  some  kinds  being  loose  and  crumbling,  others  hard 
and  crystalline.  In  many  places  it  is  composed  of  thin  layers  or 
laminae,  of  which  sixty  may  be  counted  in  the  thickness  of  an 
inch,  but  bound  together  into  a  solid  stone.  These  laminae  mark 
the  successive  layers  of  deposit.  They  are  formed  parallel  to  the 
surface  over  which  the  water  flows  or  trickles,  hence  they  may  be 
observed  not  only  on  the  flat  bottoms  of  the  pools,  but  irregularly 
enveloping  the  walls  of  the  channel  as  far  up  as  the  dash  of 
water  or  spray  can  reach.  Bounded  bosses  may  thus  be  formed 
above  the  level  of  the  stream,  and  the  recesses  may  be  hung  with 
stalactites. 

The  calcareous  springs  of  Northern  and  Central  Italy  have 
long  been  noted  for  the  large  amount  of  their  dissolved  lime,  the 
rapidity  with  which  it  is  deposited,  and  the  extensive  masses  in 
which  it  has  accumulated.  Thus  at  San  Filippo  in  Tuscany,  it 
is  deposited  in  places  at  the  rate  of  one  foot  in  four  months,  and 
it  has  been  piled  up  to  a  depth  of  at  least  250  feet,  forming  a 
hill  a  mile  and  a  quarter  long,  and  a  third  of  a  mile  broad.  So 
compact  are  many  of  the  Italian  travertines  that  they  have  from 


60  GEOLOGY 

time  immemorial  been  extensively  used  as  a  building  stone, 
which  can  be  dressed  and  is  remarkably  durable.  Many  of  the 
finest  buildings  of  ancient  and  modern  Eome  have  been  con- 
structed of  travertine. 

A  familiar  feature  of  many  calcareous  springs  deserves  notice 
here.  The  precipitation  of  calc-sinter  is  not  always  due  merely 
to  evaporation.  In  many  cases,  where  the  proportion  of  car- 
bonate of  lime  in  solution  is  so  small  that  under  ordinary  cir- 
cumstances no  precipitation  of  it  would  take  place,  large  masses 
of  it  have  been  deposited  in  a  peculiar  fibrous  form.  On  exam- 
ination, this  precipitation  will  be  found  to  be  caused  by  the  ac- 
tion of  plants,  particularly  bog-mosses  which,  decomposing  the 
carbonic  acid  in  the  water,  cause  the  lime-carbonate  to  be  de- 


Fig.  21. — Travertine  with  impressions  of  leaves. 

posited  along  their  stems  and  leaflets.  The  plants  are  thus  in- 
crusted  with  sinter  which,  preserving  their  forms,  looks  as  if  it 
were  composed  of  heaps  of  moss  turned  into  stone.  Hence  the 
name  of  petrifying  springs  often  given  to  waters  where  this  proc- 
ess is  to  be  seen.  There  is,  however,  no  true  petrifaction  or  con- 
version of  the  actual  substance  of  the  plants  into  stone.  The 
fibres  are  merely  incrusted  with  travertine,  inside  of  which  they 
eventually  die  and  decay.  But  as  the  plants  continue  to  grow 
outward,  they  increase  the  sinter  by  fresh  layers,  while  the  in- 
ner and  dead  parts  of  the  mass  are  filled  up  and  solidified  by  the 
deposit  of  the  precipitate  within  their  cavities. 


HOW  SPRINGS  LEAVE  THEIR  MARK  61 

A  growing  accumulation  of  travertine  presents  a  special 
interest  to  the  geologist  from  the  fact  that  it  offers  exceptional 
facilities  for  the  preservation  of  remains  of  the  plants  and  ani- 
mals of  the  neighborhood.  Leaves  from  the  surrounding  trees 
and  shrubs  are  blown  into  pools  or  fall  upon  moist  surfaces  where 
the  precipitation  of  lime  is  actively  going  on  (Fig.  21).  Dead 
insects,  snail-shells,  birds,  small  mammals,  and  other  denizens  of 
the  district  may  fall  or  be  carried  into  similar  positions.  These 
remains  may  be  rapidly  enclosed  within  the  stony  substance  be- 
fore they  have  time  to  decay,  and  even  if  they  should  afterwards 
moulder  into  dust,  the  sinter  enclosing  them  retains  the  mould 
of  their  forms,  and  thus  preserves  for  an  indefinite  period  the 
record  of  their  former  existence. 

Chalybeate  Springs. —  A  second  but  less  abundant  deposit  from 
springs  is  found  in  regions  where  the  rocks  below  ground  contain 
decomposing  sulphide  of  iron  (Ch.  X).  Water  percolating 
through  such  rocks  and  oxidising  the  sulphur  of  that  mineral, 
forms  sulphate  of  iron  (ferrous  sulphate)  which  it  removes  in 
solution.  The  presence  of  any  notable  quantity  of  this  sulphate 
is  at  once  revealed  by  the  marked  inky  taste  of  the  water  and 
by  the  yellowish-brown  precipitate  on  the  sides  and  bottom  of 
the  channel.  Such  water  is  termed  Chalybeate.  When  it  mixes 
with  other  water  containing  dissolved  carbonates  (which  are  so 
generally  present  in  running  water),  the  sulphate  is  decomposed, 
the  sulphuric  acid  passing  over  to  the  lime  or  alkali  of  the  car- 
bonate, while  the  iron  takes  up  oxygen  and  falls  to  the  bottom  as 
a  yellowish-brown  precipitate  (limonite,  Ch.  X).  This  inter- 
change of  combinations,  with  the  consequent  precipitation  of 
iron-oxide,  may  continue  for  a  considerable  distance  from  the 
outflow  of  the  chalybeate  water.  Nearest  the  source  the  deposit 
of  hydrated  ferric  oxide  or  ochre  is  thickest.  It  encloses  leaves, 
stems,  and  other  organic  remains,  and  preserves  moulds  or  casts 
of  their  forms.  It  also  cements  the  loose  sand  and  shingle  of  a 
river-bottom  into  solid  rock. 

Siliceous  Springs. —  One  other  deposit  from  spring-water  may 
be  enumerated  here.  In  volcanic  regions,  hot  springs  (geysers) 
rise  to  the  surface  which,  besides  other  mineral  ingredients,  con- 
tain a  considerable  proportion  of  silica  (Ch.  X).  This  substance 
is  deposited  as  Siliceous  Sinter  round  the  vents  whence  the  water 
is  discharged,  where  it  forms  a  white  stone  rising  into  mounds 


62  GEOLOGY 

and  terraces  with  fringes  and  bunches  of  coral-like  growth. 
[Where  many  springs  have  risen  in  the  same  district,  their  re- 
spective sheets  of  sinter  may  unite,  and  thus  extensive  tracts  are 
buried  under  the  deposit.  In  Iceland,  for  example,  one  of  the 
sheets  is  said  to  be  two  leagues  long,  a  quarter  of  a  league  wide, 
and  a  hundred  feet  thick.  In  the  Yellowstone  Park  of  North 
America,  many  valleys  are  floored  over  with  heaps  of  sinter,  and 
in  New  Zealand  other  extensive  accumulations  of  the  same  ma- 
terial are  to  be  found.  It  is  obvious  that,  like  travertine,  siliceous 
sinter  may  readily  entomb  and  preserve  a  record  of  the  plants 
and  animals  that  lived  at  the  time  of  its  deposition. 

SUMMARY. —  The  underground  circulation  of  water  produces 
changes  that  leave  durable  records  in  geological  history.  These 
changes  are  of  two  kinds.  (1)  Landslips  are  caused,  by  which 
the  forms  of  cliffs,  hills,  and  mountains  are  permanently  altered. 
Vast  labyrinths  of  subterranean  tunnels,  galleries,  and  caverns 
are  dissolved  out  of  calcareous  rocks,  and  openings  are  made 
from  these  passages  up  to  the  surface  whereby  rivers  are  en- 
gulfed. Many  of  the  caves  thus  hollowed  out  have  served  as 
dens  of  wild  beasts  and  dwelling-places  for  man,  and  the  relics 
of  these  inhabitants  have  been  preserved  under  the  stalagmite  of 
the  floors.  (2)  An  enormous  quantity  of  mineral  matter  is 
brought  up  to  the  surface  by  springs.  Most  of  the  solutions  are 
conveyed  ultimately  to  the  sea  where  they  partly  supply  the  sub- 
stances required  by  the  teeming  population  of  marine  plants  and 
animals.  But,  under  favourable  circumstances,  considerable  de- 
posits of  mineral  matter  are  made  by  springs,  more  especially  in 
the  form  of  travertine,  siliceous  sinter,  and  ochre.  In  these  de- 
posits the  remains  of  terrestrial  vegetation,  also  of  insects,  birds, 
mammals,  and  other  animals,  are  not  infrequently  preserved,  and 
remain  as  permanent  memorials  of  the  life  of  the  time  when  they 
flourished. 


ICE-RECORDS 


CHAPTER  VI. 

ICE-RECORDS. 

ICE  In  various  ways  alters  the  surface  of  the  land.  By  disin- 
tegrating and  eroding  even  the  most  durable  rocks,  and  by 
removing  loose  materials  and  piling  them  up  elsewhere,  it 
greatly  modifies  the  details  of  a  landscape.  As  it  assumes  various 
forms,  so  it  accomplishes  its  work  with  considerable  diversity. 
The  action  of  frost  upon  soil  and  upon  bare  surfaces  of  rock  has 
already  been  described.  We  have  now  to  consider  the 
action  of  frozen  rivers  and  lakes,  snow  and  glaciers,  which  have 
each  their  own  characteristic  style  of  operation,  and  leave  behind 
them  their  distinctive  contribution  to  the  geological  history  of 
the  earth. 

FROZEN  RIVERS  AND  LAKES. —  In  countries  with  a  severe  win- 
ter climate,  the  rivers  and  lakes  are  frozen  over,  and  the  cake  of 
ice  that  covers  them  may  be  more  than  two  feet  thick.  When 
this  cake  is  broken  up  in  early  summer,  large  masses  of  it  are 
driven  ashore,  tearing  up  the  littoral  boulders,  gravel,  sand,  or 
mud,  and  pushing  them  to  a  height  of  many  feet  above  the  ordi- 
nary level  of  the  water.  When  the  ice  melts,  huge  heaps  of  detri- 
tus are  found  to  have  been  piled  up  by  it,  which  remain  as  en- 
during monuments  of  its  power.  Not  only  so,  but  large  frag- 
ments of  the  ice  that  has  been  formed  along  shore  and  has  en- 
closed blocks  of  stone,  gravel,  and  sand,  are  driven  away  and 
may  travel  many  miles  before  they  melt  and  drop  their  freight 
of  stones.  On  the  St.  Lawrence  and  on  the  coast  of  Labrador, 
there  is  a  constant  transportation  of  boulders  by  this  means. 
Further,  besides  freezing  over  the  surface,  the  water  not  in- 
frequently forms  a  loose  spongy  kind  of  ice  on  the  bottom 
(Anchor-ice,  Ground-ice)  which  encloses  stones  and  gravel,  and 
carries  them  up  to  the  surface  where  it  joins  the  cake  of  ice 
there.  This  bottom-ice  is  formed  abundantly  on  some  parts  of 


64  GEOLOGY 

the  Canadian  rivers.  Swept  down  by  the  current,  it  accumulates 
against  the  bars  or  banks,  or  is  pushed  over  the  upper  ice,  and 
from  time  to  time  gathers  into  temporary  barriers,  the  bursting 
of  which  may  cause  destructive  floods.  In  the  river  St.  Law- 
rence, banks  and  islets  have  been  to  a  large  extent  worn  down  by 
the  grating  of  successive  ice-rafts  upon  them. 

SNOW. —  On  level  or  gently  inclined  ground,  whence  snow  dis- 
appears merely  by  melting  or  evaporation,  it  exercises,  while  it 
remains,  a  protective  influence  upon  the  soil  and  vegetation, 
shielding  them  from  the  action  of  frost.  On  slopes  of  suffi- 
cient declivity,  however,  the  sheet  of  snow  acquires  a  tendency  to 
descend  by  gravitation,  as  we  may  often  see  on  house-roofs  in 
winter.  In  many  cases,  it  creeps  or  slides  down  the  side  of  a  hill 
or  valley,  and  in  so  doing  pushes  forward  bare  soil,  loose  stones, 
or  other  objects  lying  on  the  surface.  By  this  means,  the  debris 
of  weathered  rock  in  exposed  situations  is  gradually  thrust  down- 
hill and  the  rock  is  bared  for  further  disintegration.  But  where 
the  declivities  are  steep  enough  to  allow  the  snow  to  break  off  in 
large  sheets  and  to  rush  rapidly  down,  the  most  striking  changes 
are  observable.  Such  descending  masses  are  known  as  Ava- 
lanches. Varying  from  10  to  50  feet  or  more  in  thickness  and 
several  hundred  yards  broad  and  long,  they  sweep  down  the 
mountain  sides  with  terrific  force,  carrying  away  trees,  soil, 
houses,  and  even  large  blocks  of  rock.  The  winter  of  1884-85 
was  especially  remarkable  for  the  number  of  avalanches  in  the 
valleys  of  the  Alps,  and  for  the  enormous  loss  of  life  and  proper- 
ty which  they  caused.  In  such  mountain  ground,  not  only  are 
declivities  bared  of  their  trees,  soil,  and  boulders,  but  huge 
mounds  of  debris  are  piled  up  in  the  valleys  below.  Frequently, 
also,  such  a  quantity  of  snow,  ice,  and  rubbish  is  thrown  across 
the  course  of  a  stream  as  to  dam  back  the  water,  which  accumu- 
lates until  it  overflows  or  sweeps  away  the  barrier.  In  another 
but  indirect  way,  snow  may  powerfully  affect  the  surface  of  a 
district  where,  by  rapid  melting,  it  so  swells  the  rivers  as  to  give 
rise  to  destructive  floods. 

While,  therefore,  the  influence  of  snow  is  on  the  whole  to  pro- 
tect the  surface  of  the  land,  it  shows  itself  in  mountainous 
regions  singularly  destructive,  and  leaves  as  chief  memorials  of 
this  destructiveness  the  mounds  and  rough  heaps  of  earth  and 


ICE-RECORDS 


05 


stones  that  mark  where  the  down-rushing  avalanches  have  come 
to  rest. 

GLACIERS  AND  ICE-SHEETS  leave  their  record  in  characters  so 
distinct  that  they  cannot  usually  be  confounded  with  those  of  any 
other  kind  of  geological  agent.  The  changes  which  they  produce 
on  the  surface  of  the  land  may  be  divided  into  two  parts:  (1) 
the  transport  of  materials  from  the  high  grounds  to  lower  levels, 
and  (2)  the  erosion  of  their  beds. 

(1)  Transport. —  As  a  glacier  descends  its  valley,  it  re- 
ceives upon  its  surface  the  earth,  sand,  mud,  gravel,  boulders,  and 
blocks  of  rock  that  roll  or  are  washed  down  from  the  slopes  on 
either  side.  Most  of  this  rubbish  accumulates  on  the  edges  of 


Fig.  22. —  Glacier  with  medial  and  lateral  moraines, 
the  glacier,  where  it  is  slowly  borne  to  lower  levels  as  the  ice 
creeps  downwards.  But  some  of  it  falls  into  the  crevasses  or 
rents  by  which  the  ice  is  split,  and  may  either  be  imprisoned  with- 
in the  glacier,  or  may  reach  the  rocky  floor  over  which  the  ice  is 
sliding.  The  rubbish  borne  onward  upon  the  surface  of  the 
glacier  is  known  as  moraine-stuff.  The  mounds  of  it  running 
along  each  side  of  the  glacier  form  lateral  moraines,,  those  on  the 
right-hand  side  as  we  look  down  the  length  of  the  valley  being  the 
right  lateral  moraine,  those  on  the  other  side  the  left  lateral 
moraine.  Where  two  glaciers  unite,  the  left  lateral  moraine  of 
the  one  joins  the  right  lateral  moraine  of  the  other,  forming  what 


66  GEOLOGY 

is  called  a  medial  moraine  that  runs  down  the  middle  of  the 
united  glacier.  Where  a  glacier  has  many  tributaries  bearing 
much  moraine-stuff,  its  surface  may  be  like  a  bare  plain  covered 
with  earth  and  stones,  so  that,  except  where  a  yawning  crevasse 
reveals  the  clear  blue  gleam  of  the  ice  below,  nothing  but  earth 
and  stones  meets  the  eye.  [When  the  glacier  melts,  the  detritus  is 
thrown  in  heaps  upon  the  valley,  forming  there  the  terminal 
moraine. 

Glaciers,  like  rivers,  are  subject  to  variations  of  level.  Even 
from  year  to  year  they  slowly  sink  below  their  previous  limit  or 
rise  above  it.  The  glacier  of  La  Brenva,  for  example,  on  the 
Italian  side  of  Mont  Blanc,  subsided  no  less  than  300  feet  in  the 
first  half  of  the  present  century.  One  notable  consequence  of 


Fig.  23. —  Perched  blocks  scattered  over  ice-worn  surface  of  rock. 

such  diminution  is  that  the  blocks  of  rock  lying  on  the  edges  of 
a  glacier  are  stranded  on  the  side  of  the  valley,  as  the  ice  shrinks 
away  from  them.  Such  Perched  Blocks  or  Erratics  (Fig.  23), 
as  they  are  called,  afford  an  excellent  means  of  noting  how  much 
higher  and  longer  a  glacier  has  once  been  than  it  is  now.  Their 
great  size  (some  of  them  are  as  large  as  good-sized  cottages)  and 
their  peculiar  positions  make  it  quite  certain  that  they  could  not 
have  been  transported  by  any  current  of  water.  They  are  often 
poised  on  the  tops  of  crags,  on  the  very  edges  of  precipices,  or 
on  steep  slopes  where  they  could  never  have  been  left  by  any 
flood,  even  had  the  flood  been  capable  of  moving  them.  The 
agent  that  deposited  them  in  such  positions  must  have  been  one 
that  acted  very  quietly  and  slowly,  letting  the  blocks  gently  sink 
into  the  sites  they  now  occupy.  The  only  agent  known  to  us 


ICE-RECORDS  67 

that  could  have  done  this  Is  glacier-ice.  We  can  actually  see 
similar  blocks  on  the  glaciers  now,  and  others  which  have  only 
recently  been  stranded  on  the  side  of  a  valley  from  which  the  ice 
has  sunk.  In  the  Swiss  valleys,  the  scattered  ice-borne  boulders 
may  be  observed  by  hundreds  far  above  the-  existing  level  of  the 
glaciers  and  many  miles  beyond  where  these  now  end.  If  the 
origin  of  the  dispersed  erratics  is  self-evident  in  a  valley  where  a 
glacier  is  still  busy  transporting  them,  those  that  occur  in  valleys 
which  are  now  destitute  of  glaciers  can  offer  no  difficulty;  they 
become,  indeed,  striking  monuments  that  glaciers  once  existed 
there. 

Scattered  erratic  blocks  offer  much  interesting  evidence  of  the 
movements  of  the  ice  by  which  they  were  transported.  In  a 
glacier-valley,  the  blocks  that  fall  upon  the  ice  remain  on  the  side 
from  which  they  have  descended.  Hence,  if  there  is  any  notable 
difference  between  the  rocks  of  the  two  sides,  this  difference  will 
be  recognisable  in  the  composition  of  the  moraines,  and  will 
remain  distinct  even  to  the  end  of  the  glacier.  If,  therefore,  in  a 
district  from  which  the  glaciers  have  disappeared,  we  can  trace 
up  the  scattered  blocks  to  their  sources  among  the  mountains,  we 
thereby  obtain  evidence  of  the  actual  track  followed  by  the  van- 
ished glaciers.  The  limits  to  which  these  blocks  are  traceable  do 
not,  of  course,  absolutely  fix  the  limits  of  the  ice  that  transported 
them.  They  prove,  however,  that  the  ice  extended  at  least  as  far 
as  they  occur,  but  it  may  obviously  have  risen  higher  and  ad- 
vanced farther  than  the  space  within  which  the  blocks  are  now 
confined.  In  Europe,  some  striking  examples  occur  of  the  use  of 
this  kind  of  evidence.  Thus  the  peculiar  blocks  of  the  Valais  can 
be  traced  all  the  way  to  the  site  of  the  modern  city  of  Lyons. 
There  can  therefore  be  no  doubt  that  the  glacier  of  the  Ehone 
once  extended  over  all  that  intervening  country  and  reached  at 
least  as  far  as  Lyons,—  a  distance  of  not  less  than  170  miles  from 
where  it  now  ends.  Again,  from  the  occurrence  of  blocks  of  some 
of  the  characteristic  rocks  of  Southern  Scandinavia,  in  Northern 
Germany,  Belgium,  and  the  east  of  England,  we  learn  that  a  great 
sheet  of  ice  once  filled  up  the  bed  of  the  Baltic  and  the  North 
Sea,  carrying  with  it  immense  numbers  of  northern  erratics.  In 
Britain,  where  there  are  now  neither  glaciers  nor  snow-fields,  the 
abundant  dispersion  of  boulders  from  the  chief  tracts  of  high 
ground  shows  that  this  country  was  once  in  large  part  buried 


68  GEOLOGY 

under  ice,  like  modern  Greenland.  The  evidence  for  these  state- 
ments will  be  more  fully  given  in  a  later  part  of  this  volume 
(Chapter  XXVII). 

Besides  the  moraine-stuff  carried  along  on  the  surface,  loose 
detritus  and  blocks  of  rock  are  pushed  onwards  under  the  ice. 
When  a  glacier  retires,  this  earthy  and  stony  debris,  where  not 
swept  away  by  the  escaping  river,  is  left  on  the  floor  of  the  valley. 
One  remarkable  feature  of  the  stones  in  it  is  that  a  large  pro- 
portion of  them  are  smoothed,  polished,  and  covered  with  fine 
scratches  or  ruts,  such  as  would  be  made  by  hard  sharp-pointed 
fragments  of  stone  or  grains  of  sand.  These  markings  run  for 
the  most  part  along  the  length  of  each  oblong  stone,  but  not 
infrequently  cross  each  other,  and  sometimes  an  older  may  be 
noticed  partially  effaced  by  a  newer  set.  This  peculiar  striation 


Fig.  24. —  Stone  smoothed  and  striated  by  glacier-ice. 

is  a  most  characteristic  mark  of  the  action  of  glaciers.  The 
stones  under  the  ice  are  fixed  in  the  line  of  least  resistance  —  that 
is,  end  on.  In  this  position,  under  the  weight  of  hundreds  pf  feet 
of  ice,  they  are  pressed  upon  the  floor  over  which  the  glacier  is 
travelling.  Every  sharp  edge  of  stone  or  grain  of  sand,  pressed 
along  the  surface  of  a  block,  or  over  which  the  block  itself  is 
slowly  drawn,  engraves  a  fine  scratch  or  a  deeper  rut.  As  the 
block  moves  onward,  it  is  more  and  more  scratched,  losing  its 
corners  and  edges,  and  becoming  smaller  and  smoother  till,  if  it 
travel  far  enough,  it  may  be  entirely  ground  into  sand  or  mud 
(Fig.  24). 

(2)  Erosion. —  The  same  process  of  erosion  is  carried  on  upon 
the  solid  rocks  over  which  the  ice  moves.     These  are  smoothed^ 


ICE-RECORDS 


69 


striated,  and  polished  by  the  friction  of  the  grains  of  sand,  peb- 
bles, and  blocks  of  stone  crushed  against  them  by  the  slowly 
creeping  mass  of  ice.  Every  boss  of  rock  that  looks  toward  the 
quarter  from  which  the  overlying  ice  is  moving  is  ground  away, 
while  those  that  face  to  the  opposite  side  are  more  or  less  sharp 
and  unworn.  The  striation  is  especially  noteworthy.  From  the 
fine  scratches,  such  as  are  made  by  grains  of  sand,  up  to  deep 
ruts  like  those  of  cart-wheels  in  unmended  roadways,  or  to  still 
wider  and  deeper  hollows,  all  the  friction-markings  run  in  a 
general  uniform  direction,  which  is  that  of  the  motion  of  the 
glacier.  Such  striated  surfaces  could  only  be  produced  by  some 


Fig.  25. —  Ice-striation  on  the  floor  and  side  of  a  valley. 

agent  with  rigidity  enough  to  hold  the  sand-grains  and  stones 
in  position,  and  press  them  steadily  onward  upon  the  rocks.  A 
river  polishes  the  rocks  of  its  channel  by  driving  shingle  and  sand 
across  them;  but  the  currents  are  perpetually  tossing  these 
materials  now  to  one  side  now  to  another  so  that  smoothed  and 
polished  surfaces  are  produced;  but  with  nothing  at  all  resem- 
bling striation.  A  glacier,  however,  by  keeping  its  grinding  mate- 
rials fixed  in  the  bottom  of  the  ice,  engraves  its  characteristic 
parallel  strias  and  groovings,  as  it  slowly  creeps  down  the  valley. 
All  the  surfaces  of  rock  within  reach  of  the  ice  are  smoothed,  pol- 


70  GEOLOGY 

ished,  and  striated.  Such  surfaces  present  the  most  unmistakable 
evidence  of  glacier-action,  for  they  can  be  produced  by  no  other 
known  natural  agency.  Hence,  where  they  occur  in  glacier 
valleys,  far  above  and  beyond  the  present  limits  of  the  ice,  they 
prove  how  greatly  the  ice  has  sunk.  In  regions  also  where  there 
are  now  no  glaciers,  these  rock-markings  remain  as  almost 
imperishable  witnesses  that  glaciers  once  existed.  By  means  of 
their  evidence,  for  example,  we  can  trace  the  march  of  great  ice- 
sheets  which  once  enveloped  the  whole  of  Scandinavia  and  lay 
deep  upon  nearly  the  whole  of  Britain. 

The  river  that  escapes  from  the  end  of  a  glacier  is  always 
milky  or  muddy.  The  fine  sand  and  mud  that  discolour  the 
water  are  not  supplied  by  the  thawing  of  the  clear  ice,  nor  by  the 
sparkling  brooks  that  gush  out  of  the  mountain-slopes,  nor  by  the 
melting  of  the  snows  among  the  peaks  that  rise  on  either  side. 
This  material  can  only  come  from  the  rocky  floor  of  the  glacier 
itself.  It  is  the  fine  sediment  ground  away  from  the  rocks  and 
loose  stones  by  their  mutual  friction  under  the  pressure  of  the 
overlying  ice.  It  serves  thus  as  a  kind  of  index  or  measure  of 
the  amount  of  material  worn  off  the  rocky  bed  by  the  grinding 
action  of  the  glacier.  We  can  readily  see  that  as  this  erosion 
and  transport  are  continually  in  progress,  the  amount  of  material 
removed  in  the  course  of  time  must  be  very  great.  It  has  been 
estimated,  for  example,  that  the  Justedal  glacier  in  Norway 
removes  annually  from  its  bed  2,427,000  cubic  feet  of  sediment. 
At  this  rate  the  amount  removed  in  a  century  would  be  enough  to 
fill  up  a  valley  or  ravine  10  miles  long,  100  feet  broad,  and  40 
feet  deep. 

In  arctic  and  antarctic  latitudes,  where  the  land  is  buried  under 
a  vast  ice-sheet,  which  is  continually  creeping  seaward  and  break- 
ing off  into  huge  masses  that  float  away  as  icebergs,  there  must 
be  a  constant  erosion  of  the  terrestrial  surface.  Were  the  ice  to 
retire  from  these  regions,  the  ground  would  be  found  to  wear 
what  is  called  a  glaciated  surface;  that  is  to  say,  all  the  bare 
rocks  would  present  a  characteristic  ice-worn  aspect,  rising  into 
smooth  rounded  bosses  like  dolphins'  backs  (roches  moutonnees), 
and  sinking  into  hollows  that  would  become  lake-basins.  Every- 
where these  bare  rocks  would  show  the  striae  and  groovings  graven 
upon  them  by  the  ice,  radiating  generally  from  the  central  high 
grounds,  and  thus  indicating  the  direction  of  flow  of  the  main 


ICE-RECORDS  71 

streams  of  the  ice-sheet.  Piles  of  earth,  ice-polished  stones,  and 
blocks  of  rock  would  be  found  strewn  over  the  country,  especially 
in  the  valleys  and  over  the  plains.  These  materials  would  still 
further  illustrate  the  movements  of  the  ice,  for  they  would  be 
found  to  be  singularly  local  in  character,  each  district  having 
supplied  its  own  contribution  of  detritus.  Thus  from  a  region  of 
red  sandstone,  the  rubbish  would  be  red  and  sandy;  from  one  of 
black  slate,  it  would  be  black  and  clayey  (see  Chapter  XXVI). 

SUMMARY. —  In  this  chapter  we  have  seen  that  Ice  in  various 
ways  affects  the  surface  of  the  land  and  leaves  its  mark  there. 
Frost,  as  already  explained  in  Chapter  II,  pulverises  soil,  dis- 
integrates exposed  surfaces  of  stone,  and  splits  open  bare  rocks 
along  their  lines  of  natural  joint.  On  rivers  and  lakes,  the 
disrupted  ice  wears  down  banks  and  pushes  up  mounds  of  sand, 
gravel,  and  boulders  along  the  shores.  Snow  lying  on  the  surface 
of  the  land  protects  that  surface  from  the  action  of  frost  and  air. 
In  the  condition  of  avalanches,  snow  causes  large  quantities  of 
earth,  soil,  and  blocks  of  rock  to  be  removed  from  the  mountain- 
slopes  and  piled  up  on  the  valleys.  In  the  form  of  glaciers,  ice 
transports  the  debris  of  the  mountains  to  lower  levels,  bear- 
ing along  and  sometimes  stranding  masses  of  rock  as  large  as 
cottages,  which  no  other  known  natural  agent  could  transport. 
Moving  down  a  valley,  a  glacier  wears  away  the  rocks,  giving 
them  a  peculiar  smoothed  and  striated  surface  which  is  thoroughly 
characteristic.  By  this  grinding  action,  it  erodes  its  bed  and 
produces  a  large  amount  of  fine  sediment,  which  is  carried  away 
by  the  river  that  escapes  at  the  end  of  the  ice-stream.  Land-ice 
thus  leaves  thoroughly  distinctive  and  enduring  memorials  of  its 
presence  in  polished  and  grooved  rocks,  in  masses  of  earth,  clay, 
or  gravel,  with  striated  stones,  and  in  the  dispersal  of  erratic 
blocks  from  principal  masses  of  high  ground.  These  memorials 
may  remain  for  ages  after  the  ice  itself  has  vanished.  By  their 
evidence  we  know  that  the  present  glaciers  of  the  Alps  are  only  a 
shrunk  remnant  of  the  great  ice-fields  which  once  covered  that 
region ;  that  the  Scandinavian  glaciers  swept  across  what  is  now 
the  bed  of  the  North  Sea  as  far  as  the  mouth  of  the  Thames ;  and 
that  Scotland,  Ireland,  Wales,  and  the  greater  part  of  England 
were  buried  under  great  sheets  of  ice  which  crept  downwards 
into  the  North  Sea  on  the  one  side,  and  into  the  Atlantic  on  the 
other  (Chapter  XXVII). 


72  GEOLOGY 


CHAPTER  VII. 

MEMORIALS  OF  THE  SEA. 

WE  HAVE  now  to  inquire  how  the  work  of  the  Sea  is 
registered  in  geological  history.  This  work  is  broadly 
of  two  kinds.  In  the  first  place,,  the  sea  is  engaged  in 
wearing  away  the  edges  of  the  land,  and  in  the  second  place, 
being  the  great  receptacle  into  which  all  the  materials,  worn  away 
from  the  land,  are  transported,  it  arranges  these  materials  over 
its  floor,  ready  to  be  raised  again  into  land  at  some  future  time. 

i.  DEMOLITION  OF  THE  LAND. —  In  its  work  of  destruction 
along  the  coasts  of  the  land,  the  sea  acts  to  some  extent  (though 
we  do  not  yet  know  how  far)  by  chemically  dissolving  the  rocks 
and  sediments  which  it  covers.  Cast-iron  bars,  for  example,  have 
been  found  to  be  so  corroded  by  sea-water  as  to  lose  nearly  half 
their  strength  in  fifty  years.  Doubtless  many  minerals  and  rocks 
are  liable  to  similar  attacks. 

But  it  is  by  its  mechanical  effects  that  the  sea  accomplishes 
most  of  its  erosion.  The  mere  weight  with  which  ocean-waves  fall 
upon  exposed  coasts  breaks  off  fragments  of  rock  from  cliffs. 
Masses,  13  tons  in  weight,  have  been  known  to  be  quarried  out  of 
the  solid  rock  by  the  force  of  the  breakers  in  Shetland,  at  a  height 
of  70  feet  above  sea-level.  As  a  wave  may  fall  with  a  blow  equal 
to  a  pressure  of  3  tons  on  the  square  foot,  it  compresses  the  air  in 
every  cleft  and  cranny  of  a  cliff,  and  when  it  drops  it  allows  the 
air  instantly  to  expand  again.  By  this  alternate  compression  and 
expansion,  portions  of  the  cliff  are  loosened  and  removed.  Where 
there  is  any  weaker  part  in  the  rock,  a  long  tunnel  may  be  exca- 
vated, which  may  even  be  drilled  through  to  the  daylight  above, 
forming  an  opening  at  some  distance  inland  from  the  edge  of  the 
cliff.  During  storms,  the  breakers  rush  through  such  a  tunnel, 
and  spout  forth  from  the  opening  (or  blow-hole)  in  clouds  of 
spray  (Fig.  26). 


MEMORIALS  OF  THE  SEA  73 

Probably  the  most  effective  part  of  the  destructive  action  of  the 
sea  is  to  be  found  in  the  battery  of  gravel  shingle,  and  loose  blocks 
of  stone  which  the  waves  discharge  against  cliffs  exposed  to  their 
fury.  These  loose  materials,  caught  up  by  the  advancing  breakers 
and  thrown  with  great  force  upon  the  rocks  of  a  coast-line,  are 
dragged  back  in  the  recoil  of  the  water,  but  only  to  be  again  lifted 
and  swung  forward.  In  this  loud  turmoil,  the  loose  stones  are 
reduced  in  size  and  are  ground  smooth  by  friction  against  each 
other  and  upon  the  solid  cliff.  The  well-rounded  and  polished 
aspect  of  the  gravel  on  such  storm-beaten  shores  is  an  eloquent 
testimony  to  the  work  of  the  waves.  But  still  more  striking, 


Fig.  26. —  Duller  of  Buchan  —  a  caldron-shaped  cavity  or  blow-hole  worn 
out  of  granite  by  the  sea  on  the  coast  of  Aberdeenshire. 

because  more  measurable,  is  the  proof  that  the  very  cliffs  them- 
selve  cannot  resist  the  blows  dealt  upon  them  by  the  wave-borne 
stones.  Above  the  ordinary  limit  reached  by  the  tides,  the  rocks 
rise  with  a  rough  ragged  face,  bearing  the  scars  inflicted  on  it  by 
the  ceaseless  attacks  of  the  air,  rain,  frost,  and  the  other  agencies 
that  waste  the  surface  of  the  land.  But  all  along  the  base  of  the 
cliff,  within  reach  of  the  waves,  the  rocks  have  been  smoothed  and 
polished  by  the  ceaseless  grinding  of  the  shingle  upon  them,  while 
arches,  tunnels,  solitary  pillars,  half-tide  skerries,  creeks,  and 


74  GEOLOGY 

caves  attest  the  steady  advance  of  the  sea  and  the  gradual  demoli- 
tion of  the  shore. 

Every  rocky  coast-line  exposed  to  a  tempestuous  sea  affords 
illustrations  of  these  features  of  the  work  of  waves.  Even  where 
the  rocks  are  of  the  most  durable  kind,  they  cannot  resist  the 
ceaseless  artillery  of  the  ocean.  They  are  slowly  battered  down, 
and  every  stage  in  their  demolition  may  be  witnessed,  from  the 
sunken  reef,  which  at  some  distance  from  the  shore  marks  where 
the  coast-line  once  ran,  up  to  the  tunnelled  cliff  from  which  a 
huge  mass  was  detached  during  the  storms  of  last  winter.  But 
where  the  materials  composing  the  cliffs  are  more  easily  removed, 
the  progress  of  the  waves  may  be  comparatively  rapid.  Thus  on 
the  east  coast  of  Yorkshire  between  Spurn  Point  and  Flam- 
borough  Head,  the  cliffs  consist  of  boulder-clay,  and  vary  up  to 
more  than  100  feet  in  height.  At  high  water,  the  tide  rises 
against  the  base  of  these  cliffs,  and  easily  scours  away  the  loose 
debris  which  would  otherwise  gather  there  and  protect  them. 
Hence,  within  historic  times,  a  large  tract  of  land,  with  its 
parishes,  farms,  villages,  and  seaports,  has  been  washed  away,  the 
rate  of  loss  being  estimated  at  not  less  than  2%  yards  in  a  year. 
Since  the  Roman  •  occupation  a  strip  of  land  between  2  and  3 
miles  broad  is  believed  to  have  disappeared. 

It  is  evident  that  to  carry  on  effectively  this  mechanical  erosion, 
the  sea-water  must  be  in  rapid  motion.  But  in  the  deeper  re- 
cesses of  the  ocean,  where  there  is  probably  no  appreciable  move- 
ment of  the  water,  there  can  hardly  be  any  sensible  erosion.  In 
truth  it  is  only  in  the  upper  parts  of  the  sea,  which  are  liable  to 
be  affected  by  wind,  that  the  conditions  for  marine  erosion  can 
be  said  to  exist.  The  space  within  which  these  conditions  are  to 
be  looked  for  is  that  comprised  between  the  lowest  depth  to  which 
the  influence  of  waves  and  marine  currents  extends,  and  the  great 
est  height  to  which  breakers  are  thrown  upon  the  land.  These 
limits,  no  doubt,  vary  considerably  in  different  regions.  In  some 
parts  of  the  open  sea,  as  off  the  coast  of  Florida,  the  disturbing 
action  of  the  waves  has  been  supposed  to  reach  to  a  depth  of  600 
feet,  though  the  average  limit  is  probably  greatly  less.  On  ex- 
posed promontories  in  stormy  seas,  such  as  those  .of  the  north  of 
Scotland,  breakers  have  been  known  to  hurl  up  stones  to  a  height 
of  300  feet  above  sea-level.  But  probably  the  zone,  within  which 


MEMORIALS  OF  THE  SEA 


75 


76  GEOLOGY 

the  erosive  work  of  the  sea  is  mainly  carried  on,  does  not  as  a  rule 
exceed  300  feet  in  vertical  range. 

Within  some  such  limits  as  these,  the  sea  is  engaged  in  gnaw- 
ing away  the  edges  of  the  land.  A  little  reflection  will  show  us 
that,  if  no  counteracting  operation  should  come  into  play,  the 
prolonged  erosive  action  of  the  waves  would  reduce  the  land  be- 
low the  sea-level.  If  we  suppose  the  average  rate  of  demolition  to 
be  10  feet  in  a  century,  then  it  would  take  not  less  than  52,800 
years  to  cut  away  a  strip  one  mile  broad  from  the  edge  of  the 
land.  But  while  the  sea  is  slowly  eating  away  the  coast-line, 
the  whole  surface  of  the  land  is  at  the  same  time  crumbling  down, 
and  the  wasted  materials  are  being  carried  away  by  rivers  into  the 
sea  at  such  a  rate  that,  long  before  the  sea  could  pare  away  more 
than  a  mere  narrow  selvage,  the  whole  land  might  be  worn  down 
to  the  sea-level  by  air,  rain,  and  rivers. 

But  there  are  counteracting  influences  in  nature  that  would 
probably  prevent  the  complete  demolition  of  the  land.  What 
these  influences  are  will  be  more  fully  considered  in  a  later  chap- 
ter. In  the  meantime,  it  will  be  enough  to  bear  in  mind  that 
while  the  land  is  constantly  worn  down  by  the  forces  that  are 
acting  upon  its  surface,  it  is  liable  from  time  to  time  to  be  up- 
lifted by  other  forces  acting  from  below.  And  the  existing  rela- 
tion between  the  amount  and  height  of  land,  and  the  extent  of 
sea,  on  the  face  of  the  globe,  must  be  "looked  upon  as  the  balance 
between  the  working  of  both  these  antagonistic  classes  of  agencies. 


Fig.  28. —  Section  of  submarine  plain.  I,  Land  cut  into  caves,  tunnels, 
sea-stacks,  reefs,  and  skerries  by  the  waves,  and  reduced  to  a  platform 
below  the  level  of  the  sea  (s  s)  on  which  the  gravel,  sand,  and  mud  (d) 
produced  by  the  waste  of  the  coast  may  accumulate. 

But  without  considering  for  the  present  whether  the  results  of 
the  erosion  performed  by  the  sea  will  be  interrupted  or  arrested, 
we  can  readily  perceive  that  their  tendency  is  toward  the  reduc- 
tion of  the  level  of  the  land  to  a  submarine  plain  (Fig.  28).  As 
the  waves  cut  away  slice  after  slice  from  a  coast-line,  the  portion 
of  land  which  they  thus  overflow,  and  over  which  they  drive  the 


MEMORIALS  OF  THE  SEA  77 

shingle  to  and  fro,  is  worn  down  until  it  comes  below  the  lower 
limit  of  breaker-action,  where  it  may  be  covered  up  with  sand  or 
mud.  When  the  abraded  land  has  been  reduced  to  this  level,  it 
reaches  a  limit  where  erosion  ceases,  and  where  the  sea,  no  longer 
able  to  wear  it  down  further,  protects  it  from  injury  by  other 
agents  of  demolition.  This  lower  limit  of  destruction  on  the  sur- 
face of  the  earth  has  been  termed  "  the  base-level  of  erosion." 

We  see,  then,  that  the  goal  toward  which  all  the  wear  and  tear 
of  a  coast-line  tends,  is  the  formation  of  a  more  or  less  level  plat- 
form cut  out  of  the  land.  Yet  an  attentive  study  of  the  process 
will  convince  us  that  in  the  production  of  such  a  platform  the  sea 
has  really  had  less  to  do  than  the  atmospheric  agents  of  destruc- 
tion. An  ordinary  sea-cliff  is  not  a  vertical  wall.  In  the  great 
majority  of  cases  it  slopes  seaward  at  a  steep  angle ;  but  if  it  had 
been  formed,  and  were  now  being  cut  away,  mainly  by  the  sea, 
it  ought  obviously  to  have  receded  fastest  where  the  waves  attack 
it  —  that  is,  at  its  base.  In  other  words,  if  sea-cliffs  retired  chief- 
ly because  they  are  demolished  by  the  sea,  they  ought  to  be  most 
eroded  at  the  bottom,  and  should  therefore  be  usually  overhanging 
precipices.  That  this  is  not  the  case  shows  that  some  other 
agency  is  concerned  which  causes  the  higher  parts  of  a  cliff  to 
recede  faster  than  those  below.  This  agency  can  be  no  other 
than  that  of  the  atmospheric  forces  —  air,  frost,  rain,  and 
springs.  These  cause  the  face  of  the  cliff  to  crumble  down, 
detaching  mass  after  mass,  which,  piled  up  below,  serve  as  a 
breakwater,  and  must  be  broken  up  and  removed  by  the  waves 
before  the  solid  cliff  behind  them  can  be  attacked. 

ii.  ACCUMULATIONS  FORMED  BY  THE  SEA. — It  is  not  its  erosive 
action  that  constitutes  the  most  important  claim  of  the  sea  to 
the  careful  study  of  the  geologist.  After  all,  the  mere  marginal 
belt  or  fringe  within  which  this  action  is  confined  forms  such  a 
small  fraction  of  the  whole  terrestrial  area  of  the  globe,  that  its 
importance  dwindles  down  whjen  we  compare  it  with  the  enor- 
mously vaster  surface  over  which  the  operations  of  the  air,  rain, 
rivers,  springs,  and  glaciers  are  displayed.  But  when  we  regard 
the  sea  as  the  receptacle  into  which  all  the  materials  worn  off 
the  land  ultimately  find  their  way,  we  see  what  a  large  part  it 
must  play  in  geological  history. 

During  the  last  fifteen  years  great  additions  have  been  made 
to  our  knowledge  of  the  sea-bottom  all  over  the  world.  Portions 


78  GEOLOGY 

of  the  deposits  accumulating  there  have  been  dredged  up  even 
from  the  deepest  abysses,  so  that  it  is  now  possible  to  construct 
charts,  showing  the  general  distribution  of  materials  over  the 
floor  of  the  ocean. 

Beginning  at  the  shore,  let  us  trace  the  various  types  of  marine 
deposits  outward  to  the  floors  of  the  great  ocean-abysses.  In 
many  places,  the  sea  is  more  or  less  barred  back  by  the  accumu- 
lation of  sediment  worn  away  from  the  land.  In  estuaries,  for 
example,  there  is  often  such  an  amount  of  mud  in  the  water  that 
the  bottom  on  either  side  is  gradually  raised  above  the  level  of  tide 
mark,  and  forms  eventually  a  series  of  meadows  which  the  sea 
can  no  longer  overflow.  At  the  mouths  of  rivers  with  a  consider- 
able current,  a  check  is  given  to  the  flow  of  the  water  when  it 
reaches  the  sea,  and  there  is  a  consequent  arrest  of  its  detritus. 
Hence,  a  bar  is  formed  across  the  outflow  of  a  river,  which  during 
floods  is  swept  seawards,  and  during  on-shore  gales  is  driven 
again  inland.  Even  where  there  is  no  large  river,  the  smaller 
streams  flowing  off  the  surface  of  a  country  may  carry  down 
sediment  enough  to  be  arrested  by  the  sea,  and  to  be  thrown  up 
as  a  long  bank  or  bar  running  parallel  with  the  coast.  Behind 
this  bar,  the  drainage  of  the  interior  accumulates  in  long  lagoons, 
which  find  an  outflow  through  some  breach  in  the  bar,  or  by  soak- 
ing through  the  porous  materials  of  the  bar  itself.  A  large  part 
of  the  eastern  coast  of  the  United  States  is  fringed  with  such 
bars  and  lagoons.  A  space  several  hundred  miles  long  on  the 
east  coast  of  India  is  similarly  bordered. 

But  the  most  remarkable  kind  of  accumulation  of  terrestrial 
detritus  in  the  sea  is  undoubtedly  that  of  river-deltas.  Where 
the  tidal  scour  is  not  too  great,  the  sediment  brought  down  by  a 
large  river  into  a  marine  bay  or  gulf  gradually  sinks  to  the  bot- 
tom as  the  fresh  spreads  over  and  mingles  with  the  salt  water. 
During  floods,  coarse  sediment  is  swept  along,  while  during  low 
states  of  the  river  nothing  but  fine  mud  may  be  transported.  Al- 
ternating sheets  of  different  kinds  of  sediment  are  thus  laid  down 
one  upon  another  on  the  sea-floor,  until  by  degrees  they  reacli 
the  surface,  and  thus  gradually  increase  the  breadth  of  the  land. 
Some  deltas  are  of  enormous  size  and  depth.  That  of  the  Gan- 
ges and  Brahmaputra  covers  an  area  of  between  50,000  and 
60,000  square  miles  —  that  is,  about  as  large  as  England  and 
Wales.  It  has  been  bored  through  to  a  depth  of  481  feet,  and 


MEMORIALS  OF  THE  SEA  79 

has  been  found  to  consist  of  numerous  alternations  of  fine  clays, 
marls,  and  sands  or  sandstones,  with  occasional  layers  of  gravel. 
In  all  this  great  thickness  of  sediment,  no  trace  of  marine  organ- 
isms was  found,  but  land-plants  and  bones  of  terrestrial  and 
fluviatile  animals  occurred.  Lower  Egypt  has  been  formed  by 
the  growth  of  the  delta  of  the  Nile,  whereby  a  wide  tract  of  allu- 
vial land  has  not  only  filled  up  the  bottom  of  the  valley,  but  has 
advanced  into  the  Mediterranean. 

Turning  now  to  the  deposits  that  are  more  distinctively  those 
of  the  sea  itself,  we  find  that  ridges  of  coarse  shingle,  gravel,  and 
sand  are  piled  up  along  the  extreme  upper  limit  reached  by  the 
waves.  The  coarsest  materials  are  for  the  most  part  thrown 
highest,  especially  in  bays  and  narrow  creeks  where  the  breakers 
are  confined  within  converging  shores.  In  such  situations,  during 
heavy  gales,  storm-beaches  of  coarse  rounded  shingle  are  formed 
sometimes  several  yards  above  ordinary  high-tide  mark 
(Fig.  29). 


Fig.  29. —  Storm-beach  ponding  back  a  stream  and  forming  a  lake ;  west 
coast  of  Sutherlandshire. 

Where  a  barrier  of  this  kind  is  thrown  across  the  mouth  of  a 
brook,  the  fresh  water  may  be  ponded  back  to  form  a  small  lake, 
of  which  the  outflow  usually  escapes  by  percolation  through  the 
shingle.  In  sheltered  bays,  behind  headlands,  or  on  parts  of  a 
coast-line  where  tidal  currents  meet,  detritus  may  accumulate  in 
spits  or  bars.  Islands  have  in  this  way  been  gradually  united  to 
each  other  or  to  the  mainland,  while  the  mainland  itself  has 


80  GEOLOGI 

gained  considerably  in  breadth.  At  Eomney  Marsh,  on  the 
south-east  coast  of  England,  for  instance,  a  tract  of  more  than 
80  square  miles,  which  in  lloman  times  was  in  great  part  cov- 
ered by  the  sea  at  high  water,  is  now  dry  land,  having  been 
gained  partly  by  the  natural  increase  of  shingle  thrown  up  by 
the  waves  and  partly  by  the  barriers  artificially  erected  to  exclude 
the  sea. 

While  the  coarsest  shingle  usually  accumulates  towards  the 
upper  part  of  the  beach,  the  materials  generally  arrange  them- 
selves according  to  size  and  weight,  becoming  on  the  whole  finer 
as  they  are  traced  towards  low-water  mark.  But  patches  of 
coarse  gravel  may  be  noticed  on  any  part  of  a  beach,  and  large 
boulders  may  be  seen  even  below  the  limits  of  the  lowest 
tides.  As  a  rule,  the  deposits  formed  along  a  beach,  and  in 
the  sea  immediately  beyond,  include  the  coarsest  kinds  of  marine 
sediment.  They  are  also  marked  by  frequent  alternations  of 
coarse  and  fine  detritus,  these  rapid  interchanges,  pointing  to 
the  varying  action  of  the  waves  and  strong  shore-currents.  To- 
wards the  lower  limit  of  breaker-action,  fine  gravel  and  sand 
are  allowed  to  settle  down,  and  beyond  these,  in  quiet  depths 
where  the  bottom  is  not  disturbed,  fine  sand  and  mud  washed 
away  from  the  land  slowly  accumulate. 

The  distance  to  which  the  finer  detritus  of  the  land  is  carried 
by  ocean-currents  before  it  finds  its  way  to  the  bottom,  varies 
up  to  about  200  miles  or  more.  Within  this  belt  of  sea,  the 
land-derived  materials  are  distributed  over  the  ocean-floor. 
Coarse  and  fine  gravel  and  sand  are  the  most  common  materials 
in  the  areas  nearest  the  land.  Beyond  these  lie  tracts  of  fine 
sand  and  silt  with  occasional  patches  of  gravel.  Still  farther 
from  the  land,  at  depths  of  600  feet  and  upwards,  fine  blue  and 
green  muds  are  found,  composed  of  minute  particles  of  such 
minerals  as  form  the  ordinary  rocks  of  the  land.  But  traced 
out  into  the  open  ocean,  these  various  deposits  of  recognisable 
terrestrial  origin  give  place  to  thoroughly  oceanic  accumula- 
tions, especially  to  widespread  sheets  of  exceedingly  fine  red 
and  brown  clay.  This  clay,  the  most  generally  diffused  deposit 
of  the  deeper  or  abysmal  parts  of  the  sea,  appears  to  be  derived 
from  the  decomposition  of  volcanic  fragments  either  washed 
away  from  volcanic  islands  or  supplied  by  submarine  eruptions. 
That  it  is  accumulated  with  extreme  slowness  is  shown  by  two 


MEMORIALS  OF  THE  SEA  81 

curious  and  interesting  kinds  of  evidence.  Where  it  occurs 
farthest  removed  from  land,  great  numbers  of  sharks'  teeth, 
with  ear-bones  and  other  bones  of  whales,  have  been  dredged 
up  from  it,  some  of  these  relics  being  quite  fresh,  others  partially 
coated  with  a  crust  of  brown  peroxide  of  manganese,  some  wholly 
and  thickly  enveloped  in  this  substance.  The  same  haul  of  the 
dredge  has  brought  up  bones  in  all  these  conditions,  so  that 
they  must  be  lying  side  by  side  on  the  red  clay  floor  of  the 
ocean  abysses.  The  deposition  of  manganese  is  no  doubt  an 
exceedingly  slow  process,  but  it  is  evidently  faster  than  the 
deposition  of  the  red  clay.  The  bones  dredged  up  probably 
represent  a  long  succession  of  generations  of  animals.  Yet  so 
tardily  does  the  red  clay  gather  over  them,  that  the  older  ones 
are  not  yet  covered  up  by  it,  though  they  have  had  time  to  be 
deeply  encased  in  oxide  of  manganese.  The  second  kind  of 
evidence  of  the  extreme  slowness  of  deposit  in  the  ocean  abysses 
is  supplied  by  minute  spherules  of  metallic  iron,  which  occurring 
in  numbers  dispersed  through  the  red  clay,  have  been  identified 
as  portions  of  meteorites  or  falling  stars.  These  particles  no 
doubt  fall  all  over  the  ocean,  but  it  is  only  where  the  rate  of 
deposition  of  sediment  is  exceedingly  slow  that  they  may  be 
expected  to  be  detected. 

Besides  the  sediments  now  enumerated,  the  bottom  of  the  sea 
receives  abundant  accumulations  of  the  remains  of  shells,  corals, 
foraminifera  and  other  marine  creatures;  but  these  will  be  de- 
scribed in  the  next  chapter,  where  an  account  is  given  of  the 
various  ways  in  which  plants  and  animals,  both  upon  the  land 
and  in  the  sea,  inscribe  their  records  in  geological  history.  It 
must  also  be  borne  in  mind  that  throughout  all  the  sediments 
of  the  sea-floor,  from  the  upper  part  of  the  beach  down  to  the 
bottom  of  the  deepest  and  remotest  abyss,  the  remains  of  the 
plants,  sponges,  corals,  shells,  fishes  and  other  organisms  of  the 
ocean  may  be  entombed  and  preserved.  It  will  suffice  here  to 
remember  that  various  depths  and  regions  of  the  sea  have  their 
own  characteristic  forms  of  life  the  remains  of  which  are  pre- 
served in  the  sediments  accumulating  there,  and  that  although 
gravel,  sand,  and  mud  laid  down  beneath  the  sea  may  not  differ 
in  any  recognisable  detail  from  similar  materials  deposited  in 
a  lake  or  river,  yet  the  presence  of  marine  organisms  in  them 
would  be  enough  to  prove  that  they  had  been  formed  in  the  sea. 


82  GEOLOGY 

It  is  evident,  also,  that  if  the  sea-floor  over  a  wide  area  were 
raised  into  land,  the  extent  of  the  deposits  would  show  that 
they  could  not  have  been  accumulated  in  any  mere  river  or 
lake,  but  must  bear  witness  to  the  former  presence  of  the  sea 
itself. 

SUMMARY. —  The  sea  records  its  work  upon  the  surface  of  the 
earth  in  a  twofold  way.  In  the  first  place,  in  co-operation  with 
the  atmospheric  agents  of  disintegration,  it  eats  away  the  margin 
of  the  land  and  planes  it  down.  The  final  result  of  this  process 
if  uninterrupted  would  be  to  reduce  the  level  of  the  land  to 
that  of  a  submarine  platform,  the  position  of  the  surface  of 
which  would  be  determined  by  the  lower  limit  of  effective 
breaker-action.  In  the  second  place,  the  sea  gathers  over  its 
floor  all  the  detritus  worn  by  every  agency  from  the  surface 
of  the  land.  This  material  is  not  distributed  at  random;  it  is 
assorted  and  arranged  by  the  waves  and  currents,  the  coarsest 
portions  being  laid  down  nearest  the  land,  and  the  finest  in 
stiller  and  deeper  water.  The  belt  of  sea-floor  within  which 
this  deposition  takes  place  probably  does  not  much  exceed  a 
breadth  of  200  miles.  Beyond  that  belt,  the  bottom  of  the 
ocean  is  covered  to  a  large  extent  with  deposits  of  red  clay 
derived  from  the  decomposition  of  volcanic  material  and  laid 
down  with  extreme  slowness.  These  and  the  widespread  deposits 
of  dead  sea-organisms  (to  be  described  in  next  chapter) 
are  truly  oceanic  accumulations,  recognisably  distinct  from  those 
derived  from  terrestrial  sources  within  the  narrow  zone  of  de- 
position near  the  land. 


RECORDS  OF  PLANTS  AND  ANIMALS 


CHAPTER  VIII. 

RECORDS  OF  PLANTS  AND  ANIMALS. 

BROADLY  considered,  there  are  two  distinct  ways  in  which 
Plants  and  Animals  leave  their  mark  upon  the  surface 
of  the  earth.  In  the  first  place,  they  act  directly  by 
promoting  or  arresting  the  decay  of  the  land,  and  by  forming 
out  of  their  own  remains  deposits  which  are  sometimes  thick 
and  extensive.  In  the  second  place,  their  remains  are  trans- 
ported and  entombed  in  sedimentary  accumulations  of  many 
different  kinds,  and  furnish  important  evidence  as  to  the  con- 
ditions under  which  these  accumulations  were  formed.  Each 
of  these  two  forms  of  memorial  deserves  our  careful  attention, 
for,  taken  together,  they  comprise  the  most  generally  interesting 
departments  of  geology,  and  those  in  which  the  history  of  the 
earth  is  principally  discussed. 

i.  DIRECT  ACTION  OF  LIVING  THINGS  UPON  THE  SURFACE  OF 
THE  GLOBE. —  This  action  is  often  of  a  destructive  kind,  both 
plants  and  animals  taking  their  part  in  promoting  the  general 
disintegration  of  rocks  and  soils.  Thus,  by  their  decay  they 
furnish  to  the  soil  those  organic  acids  which  have  been  referred  to 
as  so  important  in  increasing  the  solvent  power  of  water,  and 
thereby  promoting  the  waste  of  rocks.  By  thrusting  their  roots 
into  crevices  of  cliffs,  plants  loosen  and  gradually  wedge  off 
pieces  of  rock,  and  by  sending  their  roots  and  rootlets  through 
the  soil,  they  open  up  the  subsoil  to  be  attacked  by  the  air  and 
the  descending  moisture.  The  action  of  the  common  earth- 
worm in  bringing  up  fine  soil  to  be  exposed  to  the  influences 
of  wind  and  rain  has  been  referred  to.  Many  burrowing 
animals  also,  such  as  the  mole  and  rabbit,  throw  up  large  quan- 
tities of  soil  and  subsoil  which  are  liable  to  be  blown  or  washed 
away. 


84  GEOLOGY 

On  the  other  hand,  the  action  may  be  conservative,  as,  for 
instance,  where,  by  forming  a  covering  of  turf,  vegetation 
protects  the  soil  underneath  from  being  rapidly  removed,  or 
where  sand-loving  plants  bind  together  the  surface  of  dunes, 
and  thereby  arrest  the  progress  of  the  sand,  or  where  forests 
shield  a  mountain-side  from  the  effects  of  heavy  rains  and 
descending  avalanches. 

(1)  Deposits  formed  of  the  remains  of  Plants. —  But  it  is 
chiefly  by  the  aggregation  of  their  own  remains  into  more  or 
less  extensive  deposits  that  plants  and  animals  leave  their  most 
prominent  and  enduring  memorials.  As  examples  of  the  way  in 
which  this  is  done  by  plants,  reference  may  be  made  to  peat- 
bogs, mangrove-swamps,  infusorial  earth,  and  calcareous  sea- 
weeds. 

Peat-bogs. —  In  temperate  and  arctic  countries,  marshy  vege- 
tation accumulates  in  peat-bogs  over  areas  from  an  acre  or  two 
to  many  square  miles,  and  to  a  depth  of  sometimes  50  feet. 
These  deposits  are  largely  due  to  the  growth  of  bog-mosses  and 
other  aquatic  plants  which,  dying  in  their  lower  parts,  continue 
to  grow  upward  on  the  same  spot.  On  flat  or  gently-inclined 
moors,  in  hollows  between  hills,  on  valley-bottoms,  and  in  shallow 
lakes,  this  marshy  vegetation  accumulates  as  a  wet  spongy  fibrous 
mass,  the  lower  portions  of  which  by  de- 
grees become  a  more  or  less  compact 
dark  brown  or  black  pulpy  substance, 
wherein  the  fibrous  texture,  so  well  seen 
in  the  upper  or  younger  parts,  in  large 
measure  disappears.  In  a  thick  bed  of 
peat,  it  is  not  infrequently  possible  to 
detect  a  succession  of  plant  remains, 
showing  that  one  kind  of  vegetation  has 
Fig.  30. —  Section  of  a  given  place  to  another  during  the  accum- 
ulation of  the  mass.  In  Europe,  as  has 

been  already  mentioned,  peat-bogs  often  rest  directly  upon  fresh- 
water marl  containing  remains  of  lacustrine  shells  (1  in  Fig.  30). 
In  every  such  case,  it  is  evident  that  the  peat  has  accumulated 
on  the  site  of  a  shallow  lake  which  has  been  filled  up,  and  con- 
verted into  a  morass  by  the  growth  of  marsh-plants  along  its 
edges  and  over  its  floor.  The  lowest  parts  of  the  peat  may 


RECORDS  OF  PLANTS  AND  ANIMALS         85 

contain  remains  of  the  reeds,  sedges,  and  other  aquatic  plants 
which  choked  up  the  lake  (2,  3).  Higher  up,  the  peat  consists 
almost  entirely  of  the  matted  fibres  of  different  mosses,  espe- 
cially of  the  kind  known  as  Bog-moss  or  Sphagnum  (4).  The 
uppermost  layers  (5,  6)  may  be  full  of  roots  of  different  heaths 
which  spread  over  the  surface  of  the  bog. 

The  rate  of  growth  of  peat  has  been  observed  in  different 
situations  in  Central  Europe  to  vary  from  less  than  a  foot  to 
about  two  feet  in  ten  years;  but  in  more  northern  latitudes  the 
growth  is  probably  slower.  Many  thousand  square  miles  of 
Europe  and  North  America  are  covered  with  peat-bogs,  those 
of  Ireland  being  computed  to  occupy  a  seventh  part  of  the  sur- 
face of  the  island,  or  upwards  of  4000  square  miles. 

As  the  aquatic  plants  grow  from  the  sides  toward  the  centre 
of  a  shallow  lake,  they  gradually  cover  over  the  surface  of  the 
water  with  a  spongy  layer  of  matted  vegetation.  Animals,  and 
man  himself,  venturing  on  this  treacherous  surface  sink  through 
it,  and  may  be  drowned  in  the  black  peaty  mire  underneath. 
Long  afterwards,  when  the  morass  has  become  firm  ground,  and 
openings  are  made  in  it  for  digging  out  the  peat  to  be  used 
as  fuel,  their  bodies  may  be  found  in  an  excellent  state  of  preser- 
vation. The  peaty  water  so  protects  them  from  decay  that  the 
very  skin  and  hair  sometimes  remain.  In  Ireland,  numerous 
skeletons  of  the  great  Irish  elk  have  been  obtained  from  the 
bogs,  though  the  animal  itself  has  been  extinct  since  before  the 
beginning  of  the  authentic  history  of  the  country. 

Mangrove-swamps. —  Along  the  flat  shores  of  tropical  lands, 
the  mangrove  trees  grow  out  into  the  salt  water,  forming  a  belt 
of  jungle  which  runs  up  or  completely  fills  the  creeks  and  bays. 
So  dense  is  the  vegetation  that  the  sand  and  mud,  washed  into 
the  sea  from  the  land,  are  arrested  among  the  roots  and  radicles 
of  the  trees,  and  thus  the  sea  is  gradually  replaced  by  firm 
ground.  The  coast  of  Florida,  is  fringed  with  such  mangrove- 
swamps  for  a  breadth  of  from  5  to  20  miles.  In  such  regions, 
not  only  does  the  growth  of  these  swamps  add  to  the  breadth 
of  the  land,  but  the  sea  is  barred  back,  and  prevented  from 
attacking  the  newly-formed  ground  inside. 

Infusorial  earth. —  A  third  kind  of  vegetable  deposit  to  be 
referred  to  here  is  that  known  by  the  names  of  infusorial  earth, 


86 


GEOLOGY 


diatom-earth,  and  tripoli-powder.  It  consists  almost  entirely  of 
the  minute  frustules  of  microscopic  plants  called  diatoms,  which 
are  found  abundantly  in  lakes  and  likewise  in  some  region?  of 
the  ocean  (Fig.  31).  These  lowly  organisms  are  remarkable 
for  secreting  silica  in  their  structure.  As  they  die,  their  singu- 
larly durable  siliceous  remains  fall  like  a  fine  dust  on  the  bottom 
of  the  water,  and  accumulate  there  as  a  pale  gray  or  straw- 
coloured  deposit,  which,  when  dry,  is  like  flour,  and  in  its  pure 
varieties  is  made  almost  entirely  of  silica  (90  to  97  per  cent). 
Underneath  the  peat-bogs  of  Britain  a  layer  of  this  material  is 
sometimes  met  with.  One  of  the  most  famous  examples  is  that 
of  Kichmond,  Virginia,  where  a  bed  of  it  occurs  30  feet  thick. 
At  Bilin  in  Bohemia  also  an  important  bed  has  long  been  known. 
The  bottom  of  some  parts  of  the  Southern  Ocean  is  covered 


Fig.  31. —  Diatom-earth  from  floor  of  Antarctic  Ocean,  magnified  300 
diameters   (Challenger  Expedition). 

with  a  diatom-ooze  made  up  mainly  of  siliceous  diatoms,  but 
containing  also  other  siliceous  organisms  (radiolarians)  and  cal- 
careous foraminifera  (Fig.  31). 

Accumulations  of  sea-weeds. —  Yet  one  further  illustration 
of  plant-action  in  the  building  up  of  solid  rock  may  be  given. 
As  a  rule  the  plants  of  the  sea  form  no  permanent  accumulations, 
though  here  and  there  under  favourable  conditions,  such  as  in 
bays  and  estuaries,  they  may  be  thrown  up  and  buried  under 
sand  so  as  eventually  to  be  compressed  into  a  kind  of  peat.  Some 
sea-weeds,  however,  abstract  from  sea-water  carbonate  of  lime, 
which  they  secrete  to  such  an  extent  as  to  form  a  hard  stony 
structure,  as  in  the  case  of  the  common  nullipore.  When  the 
plants  die,  their  remains  are  thrown  ashore  and  pounded  up 
by  the  waves,  and  being  durable  they  form  a  white  calcareous 


RECORDS  OF  PLANTS  AND  ANIMALS  87 

sand.  By  the  action  of  the  wind,  this  sand  is  blown  inland  and 
may  accumulate  into  dunes.  But  unlike  ordinary  sand,  it  is 
liable  to  be  slightly  dissolved  by  rain-water,  and  as  the  portion 
so  dissolved  is  soon  redeposited  by  the  evaporation  of  the  mois- 
ture, the  little  sand-grains  are  cemented  together,  and  a  hard 
crust  is  formed  which  protects  the  sand  underneath  from  being 
blown  away.  Meanwhile  rain-water  percolating  through  the 
mounds  gradually  solidifies  them  by  cementing  the  particles  of 
sand  to  each  other,  and  thick  masses  of  solid  white  stone  are 
thus  produced.  Changes  of  this  kind  have  taken  place  on  a 
great  scale  at  Bermuda,  where  all  the  dry  land  consists  of  lime- 
stone formed  of  compacted  calcareous  sand,  mainly  the  detritus 
of  sea-weeds. 


Fig.  32. —  Recent  limestone  (Common  Cockle,  etc.,  cemented  in  a  matrix 
of  broken  shells). 

(2)  Deposits  formed  of  the  remains  of  Animals. —  Animals 
are,  on  the  whole,  far  more  successful  than  plants  in  leaving 
enduring  memorials  of  their  life  and  work.  They  secrete  hard 
outer  shells  and  internal  skeletons  endowed  with  great  durabil- 
ity, and  capable  of  being  piled  up  into  thick  and  extensive 
deposits  which  may  be  solidified  into  compact  and  enduring 
stone.  On  land,  we  have  an  example  of  this  kind  of  accumula- 
tion in  the  lacustrine  marl  already  described  as  formed  of  the 
congregated  remains  of  various  shells.  But  it  is  in  the  sea  that 
animals,  secreting  carbonate  of  lime,  build  up  thick  masses  of 
rock,  such  as  shell-banks,  ooze,  and  coral  reefs  (see  Chapter  XI). 

Shell-banks. —  Some  molluscs,  such  as  the  o}7ster,  live  in 
populous  communities  upon  submarine  banks.  In  the  course  of 
generations,  thick  accumulations  of  their  shells  are  formed  on 
these  banks.  By  the  action  of  currents  also  large  quantities 
of  broken  shells  are  drifted  to  various  parts  of  the  sea-bottom 


88  GEOLOGY 

not  far  from  land.  Such  deposits  of  shells,  in  situ  or  trans- 
ported, may  be  more  or  less  mixed  with  or  buried  under  sand 
and  silt,  according  as  the  currents  vary  in  direction  and  force. 
On  the  other  hand,  they  may  be  gradually  cemented  into  a 
solid  calcareous  mass,  as  has  been  observed  off  the  coast  of 
Florida,  where  they  form  on  the  sea-bottom  a  sheet  of  lime- 
stone, made  up  of  their  remains. 

Ooze. —  From  observations  made  during  the  great  expedition 
of  the  Challenger,  it  has  been  estimated  that  in  a  square  mile 
of  the  tropical  ocean  down  to  a  depth  of  100  fathoms  there 
are  more  than  16  tons  of  carbonate  of  lime  in  the  form  of 
living  animals.  A  continual  rain  of  dead  calcareous  organisms 
is  falling  to  the  bottom,  where  their  remains  accumulate  as  a 
soft  chalky  ooze.  Wide  tracts  of  the  ocean-floor  are  covered  with 
a  pale  gray  ooze  of  this  nature,  composed  mainly  of  the  remains 
of  the  shells  of  the  foraminifer  Globigerina  (Fig.  33).  In  the 


Fig.  33.  —  Globigerina  ooze  dredged  up  by  Challenger  Expedition  from  a 
depth  of  1900  fathoms  in  the  North  Atlantic 


north  Atlantic  this  deposit  probably  extends  not  less  than  1300 
miles  from  east  to  west,  and  several  hundred  miles  from  north 
to  south. 

Here  and  there,  especially  among  volcanic  islands,  portions 
of  the  sea-bed  have  been  raised  up  into  land,  and  masses  of 
modern  limestone  have  thereby  been  exposed  to  view.  Though 
they  are  full  of  the  same  kind  of  shells  as  are  still  living  in 
the  neighbouring  sea,  they  have  been  cemented  into  compact 
and  even  somewhat  crystalline  rock,  which  has  been  eaten  into 
caverns  by  percolating  water,  like  limestones  of  much  older  date. 
This  cementation,  as  above  remarked,  is  due  to  water  permeat- 
ing the  stone,  dissolving  from  its  outer  parts  the  calcareous 


RECORDS  OF  PLANTS  AND  ANIMALS         89 

matter  of  shells,  corallines,  and  other  organic  remains,  and 
redepositing  it  again  lower  down,  so  as  to  cement  the  organic 
detritus  into  a  compact  stone. 

Coral-reefs  offer  an  impressive  example  of  how  extensive 
masses  of  solid  rock  may  be  built  up  entirely  of  the  aggregated 
remains  of  animals.  In  some  of  the  warmer  seas  of  the  globe, 
and  notably  in  the  track  of  the  great  ocean-currents,  where 
marine  life  is  so  abundant,  various  kinds  of  coral  take  root 
upon  the  edges  and  summits  of  submerged  ridges  and  peaks, 
as  well  as  on  the  shelving  sea-bottom  facing  continents  or 
encircling  islands  (1  in  Fig.  34).  These  creatures  do  not  appear 
to  nourish  at  a  greater  depth  than  15  or  20  fathoms,  and  they 
are  killed  by  exposure  to  sun  and  air.  The  vertical  space  within 
which  they  live  may  therefore  be  stated  broadly  as  about  100 
feet.  They  grow  in  colonies,  each  composed  of  many  indivi- 
duals, but  all  united  into  one  mass,  which  at  first  may  be 
merely  a  little  solitary  clump  on  the  sea-floor,  but  which,  as 
it  grows,  joins  other  similar  clumps  to  form  what  is  known  as 


Fig.  34. —  Section  of  a  coral-reef.  1.  Top  of  the  submarine  ridge  or 
bank  on  which  the  corals  begin  to  build.  2.  Coral-reef.  3.  Talus  of 
large  blocks  of  coral-rock  on  which  the  reef  is  built  outward.  4.  Fine 
coral  sand  and  mud  produced  by  the  grinding  action  of  the  breakers 
on  the  edge  of  the  reef.  5.  Coral  sand  thrown  up  by  the  waves 
and  gradually  accumulating  above  their  reach  to  form  dry  ground. 

a  reef.  Each  individual  secretes  from  the  sea-water  a  hard 
calcareous  skeleton  inside  its  transparent  jelly-like  body,  and 
when  it  dies,  this  skeleton  forms  part  of  the  platform  upon 
which  the  next  generation  starts.  Thus  the  reef  is  gradually 
built  upward  as  a  mass  of  calcareous  rock  (2),  though  only  its 
upper  surface  is  covered  with  living  corals.  These  creatures 
continue  to  work  upward  until  they  reach  low-water  mark,  and 
then  their  further  upward  progress  is  checked.  But  they  are 
still  able  to  grow  outward.  On  the  outer  edges  of  the  reef  they 
flourish  most  vigorously,  for  there,  amid  the  play  of  the  breakers, 
they  find  the  food  that  is  brought  to  them  by  the  ocean-currents. 


90  GEOLOGY 

From  time  to  time  fragments  are  torn  off  by  breakers  from 
the  reef  and  roll  down  its  steep  front  (3).  There,  partly  by 
the  chemical  action  of  the  sea-water,  and  partly  by  the  fine 
calcareous  mud  and  sand  (4),  produced  by  the  grinding  action 
of  the  waves  and  washed  into  their  crevices,  these  loose  blocks 
are  cemented  into  a  firm,  steep  slope,  on  the  top  of  which  the 
reef  continues  to  grow  outwards.  Blocks  of  coral  and  quan- 
tities of  coral-sand  are  also  thrown  up  on  the  surface  of  the 
reef,  where  by  degrees  they  form  a  belt  of  low  land  above  the 
reach  of  the  waves  (5).  On  the  inside  of  the  reef,  where  the 
corals  cannot  find  the  abundant  food-supply  afforded  by  the 
open  water  outside,  they  dwindle  and  die.  Thus  the  tendency 
of  all  reefs  must  be  to  grow  seawards,  and  to  increase  in  breadth. 
Perhaps  their  breadth  may  afford  some  indication  of  their  rela- 
tive age. 

Where  a  reef  has  started  on  a  shelving  sea-bottom  near  the 
coast  of  a  continent,  or  round  a  volcanic  island,  the  space  of 
water  inside  is  termed  the  Lagoon  Channel.  Where  the  reef 
has  been  built  up  on  some  submarine  ridge  or  peak,  and  there 
is  consequently  no  land  inside,  the  enclosed  space  of  water  is 
called  a  Lagoon,  and  the  circular  reef  of  coral  is  known  as  an 
Atoll.  If  no  subsidence  of  the  sea-bottom  takes  place,  the  maxi- 
mum thickness  of  a  reef  must  be  limited  by  the  space  within 
which  the  corals  can  thrive  —  that  is,  a  vertical  depth  of  about 
100  feet  from  the  surface  of  the  sea.  But  the  effect  of  the 
destruction  of  the  ocean-front  of  the  reef,  and  the  piling  up 
of  a  slope  of  its  fragments  on  the  sea-bottom  outside,  will  be 
to  furnish  a  platform  of  the  same  materials  on  which  the  reef 
itself  may  grow  outward,  so  that  the  united  mass  of  calcareous 
rock  may  attain  a  very  much  greater  thickness  than  100  feet. 
On  the  other  hand,  if  the  sea-bottom  were  to  sink  at  so  slow 
a  rate  that  the  reef-building  corals  could  keep  pace  with  the 
subsidence,  a  mass  of  calcareous  rock  many  thousand  feet  thick 
might  obviously  be  formed  by  them.  It  is  a  disputed  question 
in  which  of  these  two  ways  atolls  have  been  formed. 

It  is  remarkable  how  rapidly  and  completely  the  structure 
of  the  coral-skeleton  is  effaced  from  the  coral-rock,  and  a  more 
or  less  crystalline  and  compact  texture  is  put  in  its  place.  The 
change  is  brought  about  partly  by  the  action  of  both  sea-water 
and  rain-water  in  dissolving  and  redepositing  carbonate  of  lime 


RECORDS  OF  PLANTS  AND  ANIMALS         91 

among  the  minute  interstices  of  the  rock,  and  partly  also  by 
the  abundant  mud  and  sand  produced  by  the  pounding  action 
of  the  breakers  on  the  reef,  and  washed  into  the  crevices.  On 
the  portion  of  a  reef  laid  dry  at  low  water,  the  coral-rock  looks 
in  many  places  as  solid  and  old  as  some  of  the  ancient  white 
limestones  and  marbles  of  the  land.  There,  in  pools  where  a 
current  or  ripple  of  water  keeps  the  grains  of  coral-sand  in 
motion,  each  grain  may  be  seen  to  have  taken  a  spherical  form 
unlike  that  of  the  ordinary  irregularly  rounded  or  angular 
particles.  This  arises  because  carbonate  of  lime  in  solution 
in  the  water  is  deposited  round  each  grain  as  it  moves  along. 
A  mass  of  such  grains  aggregated  together  is  called  oolite,  from 
its  resemblance  to  fish-roe.  In  many  limestones,  now  forming 
wide  tracts  of  richly  cultivated  country,  this  oolitic  structure 
is  strikingly  exhibited.  There  can  be  no  doubt  that  in  these 
cases  it  was  produced  in  a  similar  way  to  that  now  in  progress 
on  coral-reefs. 

In  the  coral  tracts  of  the  Pacific  Ocean  there  are  nearly  300 
coral  islands,  besides  extensive  reefs  round  volcanic  islands. 
Others  occur  in  the  Indian  Ocean.  Coral-reefs  abound  in  the 
West  Indian  Seas,  where,  on  many  of  the  islands,  they  have 
been  upraised  into  dry  land,  in  Cuba  to  a  height  of  1100  feet 
above  sea-level.  The  Great  Barrier  Eeef  that  fronts  the  north- 
eastern coast  of  Australia  is  1250  miles  long,  and  from  10  to 
90  miles  broad. 

There  are  other  ways  in  which  the  aggregation  of  animal 
remains  forms  more  or  less  extensive  and  durable  rocks.  To 
some  of  these  reference  will  be  made  in  later  chapters.  Enough 
has  been  said  here  to  show  that  by  the  accumulation  of  their 
hard  parts  animals  leave  permanent  records  of  their  presence 
both  on  land  and  in  the  sea. 

ii.  PRESERVATION  OF  REMAINS  or  PLANTS  AND  ANIMALS  IN 
SEDIMENTARY  DEPOSITS. — But  it  is  not  only  in  rocks  formed 
out  of  their  remains  that  living  things  leave  their  enduring 
records.  These  remains  may  be  preserved  in  almost  every  kind 
of  deposit,  under  the  most  wonderful  variety  of  conditions.  And 
as  it  is  in  large  measure  from  their  occurrence  in  such  deposits 
that  the  geologist  derives  the  evidence  that  successive  tribes 
of  plants  and  animals  have  peopled  the  globe,  and  that  the 
climate  and  geography  of  the  earth  have  greatly  varied  at 


92  GEOLOGY 

different  periods,  we  shall  find  it  useful  to  observe  the  different 
ways  in  which  the  remains  both  of  plants  and  animals  are  at 
this  moment  being  entombed  and  preserved  upon  the  land  and 
in  the  sea.  With  the  knowledge  thus  gained,  it  will  be  easier 
to  understand  the  lessons  taught  by  the  organic  remains  that 
lie  among  the  various  solid  rocks  around  us. 

It  is  evident  that  in  the  vast  majority  of  cases,  the  plants 
and  animals  of  the  land  leave  no  perceptible  trace  of  their  pres- 
ence. Of  the  forests  that  once  covered  so  much  of  Central  and 
Northern  Europe,  which  is  now  bare  ground,  most  have  disap- 
peared, and  unless  authentic  history  told  that  they  had  once 
flourished,  we  should  never  have  known  anything  about  them. 
There  were  also  herds  of  wild  oxen,  bears,  wolves,  and  other 
denizens  contemporaneous  with  the  vanished  forests.  But  they 
too  have  passed  away,  and  we  might  ransack  the  soil  in  vain 
for  any  trace  of  them. 

If  the  remains  of  terrestrial  vegetation  and  animals  are  any- 
where preserved  it  must  obviously  be  only  locally,  but  the  favour- 
able circumstances  for  their  preservation,  although  not  every- 
where to  be  found,  do  present  themselves  in  many  places  if 
we  seek  for  them.  The  fundamental  condition  is  that  the  relics 
should,  as  soon  as  possible  after  death,  be  so  covered  up  as 
to  be  protected  from  the  air  and  from  too  rapid  decomposition. 
Where  this  condition  is  fulfilled,  the  more  durable  of  them  may 
be  preserved  for  an  indefinite  series  of  ages. 

(a)  On  the  Land  there  are  various  places  where  the  remains 
both  of  plants  and  animals  are  buried  and  shielded  from  decay. 
To  some  of  these  reference  has  already  been  made.  Thus  amid 
the  fine  silt,  mud,  and  marl  gathering  on  the  floors  of  lakes, 
leaves,  fruits,  and  branches,  or  tree-trunks,  washed  from  the 
neighbouring  shores,  may  be  imbedded,  together  with  insects, 
birds,  fishes,  lizards,  frogs,  field-mice,  rabbits,  and  other  in- 
habitants. These  remains  may  of  course  often  decay  on  the 
lake-bottom,  but  where  they  sink  into  or  are  quickly  covered 
up  by  the  sediment,  they  may  be  effectually  preserved  from 
obliteration.  They  undergo  a  change,  indeed,  being  gradually 
turned  into  stone,  as  will  be  described  in  Chapter  XV.  But 
this  conversion  may  be  effected  so  gently  as  to  retain  the  finest 
microscopic  textures  of  the  original  organisms. 

In  peat-bogs  also,  as  stated  elsewhere,  wild  animals  are  often 


RECORDS  OF  PLANTS  AND  ANIMALS         93 

engulfed,  and  their  soft  parts  are  occasionally  preserved  as 
well  as  their  skeletons.  The  deltas  of  river-mouths  must  re- 
ceive abundantly  the  remains  of  animals  swept  off  by  floods. 
As  the  carcases  float  seawards,  they  begin  to  fall  to  pieces  and 
the  separate  bones  sink  to  the  bottom,  where  they  are  soon 
buried  in  the  silt.  Among  the  first  bones  to  separate  from 
the  rest  of  the  skeleton  are  the  lower  jaws  (see  Ch.  XXIII) .  We 
should  therefore  expect  that  in  excavations  made  in  a  delta  these 
bones  would  occur  most  frequently.  The  rest  of  the  skeleton 
is  apt  to  be  carried  farther  out  to  sea  before  it  can  find  its  way 
to  the  bottom.  The  stalagmite  floor  of  caverns  has  already  been 
referred  to  (Ch.  V)  as  an  admirable  material  for  enclosing  and 
preserving  organic  remains.  The  animals  that  fell  into  these 
recesses,  or  used  them  as  dens  in  which  they  lived  or  into  which 
they  dragged  their  prey,  have  left  their  bones  on  the  floors, 
where  encased  in  or  covered  by  solid  stalagmite,  these  relics 
have  remained  for  ages.  Most  of  our  knowledge  of  the  animals 
which  inhabited  Europe  at  the  time  when  man  appeared,  is 
derived  from  the  materials  disinterred  from  these  Bone-caves. 
Allusion  has  also  been  made  to  the  travertine  formed  by  mineral- 
springs  and  to  the  facility  with  which  leaves,  shells,  insects,  and 
small  birds,  reptiles,  or  mammals  may  be  enclosed  and  pre- 
served in  travertine.  Thus,  while  the  plants  and  animals  of 
the  land  for  the  most  part  die  and  decay  into  mere  mould,  there 
are  here  and  there  localities  where  their  remains  axe  covered 
up  from  decay  and  preserved  as  memorials  of  the  life  of  the 
time. 

(&)  On  the  bottom  of  the  Sea  the  conditions  for  the  pres- 
ervation of  organic  remains  are  more  general  and  favourable 
than  on  land.  Among  the  sands  and  gravels  of  the  shore,  some 
of  the  stronger  shells  that  live  in  the  shallower  waters  near 
land  may  be  covered  up  and  preserved,  though  often  only  in 
rolled  fragments.  It  is  below  tide-mark,  however,  and  more 
especially  beneath  the  limit  to  which  the  disturbing  action  of 
breakers  descends,  that  the  remains  of  the  denizens  of  the  sea 
are  most  likely  to  be  buried  in  sediment  and  to  be  preserved 
there  as  memorials  of  the  life  of  the  sea.  It  is  evident  that 
hard  and  therefore  durable  relics  have  the  best  chance  of  escap- 
ing destruction.  Shells,  corals,  corallines,  spicules  of  sponges, 
teeth,  vertebra,  and  ear-bones  of  fishes  may  be  securely  entombed 


94  GEOLOGY 

in  successive  layers  of  silt  or  mud.  But  the  vast  crowds  of 
marine  creatures  that  have  no  hard  parts  must  almost  always 
perish  without  leaving  any  trace  whatever  of  their  existence. 
And  even  in  the  case  of  those  which  possess  hard  shells  or  skele- 
tons, it  will  be  easily  understood  that  the  great  majority  of  them 
must  be  decomposed  upon  the  sea-bottom,  their  component  ele- 
ments passing  back  again  into  the  sea-water  from  which  they 
were  originally  derived.  It  is  only  where  sediment  is  deposited 
fast  enough  to  cover  them  up  and  protect  them  before  they  have 
time  to  decay,  that  they  may  be  expected  to  be  preserved. 

In  the  most  favourable  circumstances,  therefore,  only  a  very 
small  proportion  of  the  creatures  living  in  the  sea  at  any  time 
leave  a  tangible  record  of  their  presence  in  the  deposits  of  the  sea- 
bottom.  It  is  in  the  upper  waters  of  the  ocean,  and  especially 
in  the  neighbourhood  of  land,  that  life  is  most  abundant.  The 
same  region  also  is  that  in  which  the  sediment  derived  from  the 
waste  of  the  land  is  chiefly  distributed.  Hence  it  is  in  these 
marginal  parts  of  the  ocean  that  the  conditions  for  preserving 
memorials  of  the  animals  that  inhabit  the  sea  are  best  developed. 

As  we  recede  from  the  land,  the  rate  of  deposit  of  sediment  on 
the  sea-floor  gradually  diminishes,  until  in  the  central  abysses  it 
reaches  that  feeble  stage  so  strikingly  brought  before  us  by  the 
evidence  of  the  manganese  nodules  (Ch.  VII).  The  larger  and 
thinner  calcareous  organisms  are  attacked  by  the  sea-water  and 
dissolved,  apparently  before  they  can  sink  to  the  bottom;  at 
least  their  remains  are  comparatively  rarely  found  there.  It  is 
such  indestructible  objects  as  sharks'  teeth  and  vertebrae  and  ear- 
bones  of  whales  that  form  the  most  conspicuous  organic  relics 
in  these  abysmal  deposits. 

SUMMARY. —  Plants  and  animals  leave  their  records  in  geo- 
logical history,  partly  by  forming  distinct  accumulations  of  their 
remains,  partly  by  contributing  their  remains  to  be  imbedded  in 
different  kinds  of  deposits  both  on  land  and  in  the  sea.  As 
examples  of  the  first  mode  of  chronicling  their  existence,  we 
may  take  the  growth  of  marsh-plants  in  peat-bogs,  the  spread 
of  mangrove-swamps  along  tropical  shores,  and  the  deposition 
of  infusorial  earth  on  the  bottom  of  lakes  and  of  the  sea;  the 
accumulation  of  nullipore  sand  into  solid  stone,  the  formation 
of  extensive  shell-banks  in  many  seas,  the  wide  diffusion  of 
organic  ooze  over  the  floor  of  the  sea,  and  the  growth  of  coral 


RECORDS  OF  PLANTS  AND  ANIMALS  95 

reefs.  As  illustrations  of  the  second  method,  we  may  cite  the 
manner  in  which  the  remains  of  terrestrial  plants  and  animals 
are  preserved  in  peat-bogs,  in  the  deltas  of  rivers,  in  the  stalag- 
mite of  caverns,  and  in  the  travertine  of  springs ;  and  the  way  in 
which  the  hard  parts  of  marine  creatures  are  entombed  in  the 
sediments  of  the  sea-floor,  more  especially  along  that  belt  fring- 
ing the  continents  and  islands,  where  the  chief  deposit  of  sedi- 
ment from  the  disintegration  of  the  land  takes  place.  Never- 
theless, alike  on  land  and  sea,  the  proportion  of  organic  remains 
thus  sealed  up  and  preserved  is  probably  always  but  an  insignifi- 
cant part  of  the  total  population  of  plants  and  animals  living  at 
any  given  moment. 

How  the  remains  of  plants  and  animals  when  once  entombed 
in  sediment  are  then  hardened  and  petrified,  so  as  to  retain  their 
minute  structures,  and  to  be  capable  of  enduring  for  untold 
ages,  will  be  treated  of  in  Chapter  XV. 


96  GEOLOGY 


CHAPTER  IX. 

VOLCANOES  AND  EARTHQUAKES. 

THE  geological  changes  described  in  the  foregoing  chapters 
affect  only  the  surface  of  the  earth.  A  little  reflection 
will  convince  us  that  they  may  all  be  referred  to  one  com- 
mon source  of  energy  —  the  sun.  It  is  chiefly  to  the  daily  influ- 
ence of  that  great  centre  of  heat  and  light  that  we  must  ascribe 
the  ceaseless  movements  of  the  atmosphere,  the  phenomena 
of  evaporation  and  condensation,  the  circulation  of  water  over 
the  land,  the  waves  and  currents  of  the  sea,  in  short  the  whole 
complex  s}>-stem  which  constitutes  what  has  been  called  the  Life 
of  the  Earth.  Could  this  influence  be  conceivably  withdrawn, 
the  planet  would  become  cold,  dark,  silent,  lifeless. 

But  besides  the  continual  transformations  of  its  surface  due 
to  solar  energy,  our  globe  possesses  distinct  energy  of  its  own. 
Its  movements  of  rotation  and  revolution,  for  example,  provide 
a  vast  store  of  force,  whereby  many  of  the  most  important  geo- 
logical processes  are  initiated  or  modified,  as  in  the  phenomena 
of  day  and  night  and  the  seasons,  with  the  innumerable  me- 
teorological and  other  effects  that  flow  therefrom.  These  move- 
ments, though  slowly  growing  feebler,  bear  witness  to  the  won- 
derful vigour  of  the  earlier  phases  of  the  earth's  existence.  In- 
side the  globe  too  lies  a  vast  magazine  of  planetary  energy  in 
the  form  of  an  interior  of  intensely  hot  material.  The  cool 
outer  shell  is  but  an  insignificant  part  of  the  total  bulk  of  the 
globe.  To  this  cool  part  the  name  of  "  crust "  was  given  at  a 
time  when  the  earth  was  believed  to  consist  of  an  inner  molten 
nucleus  enclosed  within  an  outer  solid  shell  or  crust.  The  term 
is  now  used  merely  to  denote  the  cool  solid  external  part  of  the 
globe,  without  implying  any  theory  as  to  the  nature  of  the  in- 
terior. 


VOLCANOES  AND  EARTHQUAKES  97 

CONDITION  OF  THE  EARTH'S  INTERIOR.: —  It  is  obvious  that  we 
are  not  likely  ever  to  learn  by  direct  observation  what  may  be  the 
condition  of  the  interior  of  our  planet.  The  cool  solid  outer 
shell  is  far  too  thick  to  be  pierced  through  by  human  efforts; 
but  by  various  kinds  of  observations,  more  or  less  probable  con- 
clusions may  be  drawn  with  regard  to  this  problem.  In  the  first 
place,  it  has  been  ascertained  that  all  over  the  world,  wherever 
borings  are  made  for  water  or  in  mining  operations,  the  temper- 
ature increases  in  proportion  to  the  depth  pierced,  and  that  the 
average  rate  of  increase  amounts  to  about  one  degree  Fahrenheit 
for  every  64  feet  of  descent.  If  the  rise  of  temperature  con- 
tinues inward  at  this  rate,  or  at  any  rate  at  all  approaching  it, 
then  at  a  distance  from  the  surface,  which  in  proportion  to  the 
bulk  of  the  whole  globe  is  comparatively  trifling,  the  heat  must 
be  as  great  as  that  at  which  the  ordinary  materials  of  the  crust 
would  melt  at  the  surface.  In  the  second  place,  thermal  springs 
in  all  quarters  of  the  globe,  rising  sometimes  with  the  temper- 
ature of  boiling  water,  and  occasionally  even  still  hotter,  prove 
that  the  interior  of  the  planet  must  be  very  much  warmer  than 
its  exterior.  In  the  third  place,  volcanoes  widely  distributed 
over  the  earth's  surface  throw  out  steam  and  heated  vapours, 
red-hot  stones,  and  streams  of  molten  rock. 

It  is  quite  certain  therefore  that  the  interior  of  the  globe 
must  be  intensely  hot ;  but  whether  it  is  actually  molten  or  solid 
has  been  the  subject  of  prolonged  discussion.  Three  opinions 
have  found  stout  defenders.  (1)  The  older  geologists  main- 
tained that  the  phenomena  of  volcanoes  and  earthquakes  could 
not  be  explained,  except  on  the  supposition  of  a  crust  only  a 
few  miles  thick,  enclosing  a  vast  central  ocean  of  molten  ma- 
terial. (2)  This  view  has  been  opposed  by  physicists  who  have 
shown  that  the  globe,  if  this  were  actually  its  structure,  could  not 
resist  the  attraction  of  sun  and  moon,  but  would  be  drawn 
out  of  shape,  as  the  ocean  is  in  the  phenomenon  of  the  tides, 
and  that  the  absence  of  any  appreciable  tidal  deformation  in  the 
crust  shows  that  the  earth  must  be  practically  solid  and  as  rigid 
as  a  ball  of  glass,  or  of  steel.  (3)  A  third  opinion  has  been 
advanced  by  geologists  who,  while  admitting  that  the  earth 
behaves  on  the  whole  as  a  solid  rigid  body,  yet  believe  that  many 
geological  phenomena  can  only  be  explained  by  the  existence 
of  some  liquid  mass  beneath  the  crust.  Accordingly  they  sup- 


98  GEOLOGY 

pose  that  while  the  nucleus  is  retained  in  the  solid  state  by  the 
enormous  superincumbent  pressure  under  which  it  lies,  and  the 
crust  has  become  solid  by  cooling,  there  is  an  intermediate  liquid 
or  viscous  layer  which  has  not  yet  cooled  sufficiently  to  pass  into 
the  solid  crust  above,  and  does  not  lie  under  sufficient  pressure 
to  form  part  of  the  solid  nucleus  below.  At  present,  the  balance 
of  evidence  and  argument  seems  to  be  in  favour  of  the  practical 
rigidity  and  solidity  of  the  globe  as  a  whole.  But  the  materials 
of  its  interior  must  possess  temperatures  far  higher  than  those 
at  which  they  would  melt  at  the  surface.  They  are  no  doubt 
kept  solid  by  the  vast  overlying  pressure,  and  any  change  which 
could  relieve  them  of  this  pressure  would  allow  them  to  pass  into 
the  liquid  form.  This  subject  will  be  again  alluded  to  in  Chap- 
ter XVI.  Meanwhile,  let  us  consider  how  the  intensely  hot 
nucleus  of  the  planet  reacts  upon  its  surface. 

Eocks  are  bad  conductors  of  heat.  So  slowly  is  the  heat  of 
the  interior  conducted  upwards  by  them  that  the  temperature 
of  the  surface  of  the  crust  is  not  appreciably  affected  by  that  of 
the  intensely  hot  nucleus.  But  the  fact  that  the  surface  is  not 
warmed  from  this  source  shows  that  the  heat  of  the  interior 
must  pass  off  into  space  as  fast  as  it  arrives  at  the  surface,  and 
proves  that  our  planet  is  gradually  cooling.  For  many  millions 
of  years  the  earth  has  been  radiating  heat  into  space,  and  has 
consequently  been  losing  energy.  Its  present  store  of  planetary 
vitality  therefore  must  be  regarded  as  greatly  less  than  it  once 
was. 

VOLCANOES. 

Of  all  the  manifestations  of  this  planetary  vitality,  by  far  the 
most  impressive  are  those  furnished  by  volcanoes.  The  general 
characters  of  these  vents  of  communication  between  the  hot 
interior  and  cool  surface  of  the  planet  are  doubtless  already 
familiar  to  the  reader  of  these  chapters  —  the  volcano  itself,  a 
conical  hill  or  mountain,  formed  mainly  or  entirely  of  materials 
ejected  from  below,  having  on  its  truncated  summit  the  basin- 
shaped  crater,  at  the  bottom  of  which  lies  the  vent  or  funnel 
from  which,  as  well  as  from  rents  on  the  flanks  of  the  cone, 
hot  vapours,  cinders,  ashes,  and  streams  of  molten  lava  are 
discharged,  till  they  gradually  pile  up  the  volcanic  cone  round 
the  vent  whence  they  escape. 


VOLCANOES  AND  EARTHQUAKES  99 

A  volcanic  cone,  so  long  as  it  remains,  bears  eloquent  testi- 
mony to  the  nature  of  the  causes  that  produced  it.  Even  many 
centuries  after  it  has  ceased  to  be  active,  when  no  vapours  rise 
from  any  part  of  its  cold,  silent,  and  motionless  surface,  its 
conical  form,  its  cup-shaped  crater,  its  slopes  of  loose  ashes,  and 
its  black  bristling  lava-currents  remain  as  unimpeachable  wit- 
nesses that  the  volcanic  fires,  now  quenched,  once  blazed  forth 
fiercely.  The  wonderful  groups  of  volcanoes  in  Auvergne  and 
the  Eifel  are  as  fresh  as  if  they  had  not  yet  ceased  to  be  active, 
and  might  break  forth  again  at  any  moment ;  yet  they  have  been 
quiescent  ever  since  the  beginning  of  authentic  human  history. 

But  in  the  progress  of  the  degradation  which  everywhere 
slowly  changes  the  face  of  the  land,  it  is  impossible  that  vol- 
canic hills  should  escape  the  waste  which  befalls  every  other 
kind  of  eminence.  We  can  picture  a  time  when  the  volcanic 
cones  of  Auvergne  will  have  been  worn  away,  and  when  the 
lava-streams  that  descend  from  them  will  be  cut  into  ravines 
and  isolated  into  separate  masses  by  the  streams  that  have  even 
already  deeply  trenched  them.  Where  all  the  ordinary  and 
familiar  signs  of  a  volcano  have  been  removed,  how  can  we  tell 
that  any  volcano  ever  existed?  What  enduring  record  do  vol- 
canoes inscribe  in  geological  history? 

Now,  it  must  be  obvipus  that  among  the  operations  of  an 
active  volcano,  many  of  the  most  striking  phenomena  have 
hardly  any  importance  as  aids  in  recognising  the  traces  of  long- 
extinct  volcanic  action.  The  earthquakes  and  tremors  that  ac- 
company volcanic  outbursts,  the  constant  and  prodigious  out- 
rushing  of  steam,  the  abundant  discharge  of  gases  and  acid 
vapours,  though  singularly  impressive  at  the  time,  leave  little 
or  no  lasting  mark  of  their  occurrence.  It  is  not  in  phenomena, 
so  to  speak,  transient  in  their  effects,  that  we  must  seek  for  a 
guide  in  exploring  the  records  of  ancient  volcanoes,  but  in  those 
which  fracture  or  otherwise  affect  the  rocks  below  ground,  and 
pile  up  heaps  of  material  above. 

Keeping  this  aim  before  us,  we  may  obtain  from  an  examina- 
tion of  what  takes  place  at  an  active  volcano  such  durable  proofs 
of  volcanic  energy  as  will  enable  us  to  recognise  the  former  ex- 
istence of  volcanoes  over  many  tracts  of  the  globe  where  human 
eye  has  never  witnessed  an  eruption,  and  where,  indeed,  all  trace 
of  what  could  be  called  a  volcano  has  utterly  vanished.  A 


100  GEOLOGY 

method  of  observation  and  reasoning  has  been  established,  from 
the  use  of  which  we  learn  that  in  some  countries,  Britain  for  ex- 
ample, though  there  is  now  no  sign  of  volcanic  activity,  there  has 
been  a  succession  of  volcanoes  during  many  protracted  and 
widely  separated  periods,  and  that  probably  the  interval  that  has 
passed  away  since  the  last  eruptions  is  not  so  vast  as  that  which 
separated  these  from  those  that  preceded  them.  A  similar 
story  has  been  made  out  in  many  parts  of  the  continent  of  Eu- 
rope, in  the  United  States,  India,  and  New  Zealand,  and,  in- 
deed, in  most  countries  where  the  subject  has  been  fully  investi- 
gated. 

A  little  reflection  on  this  question  will  convince  us  that  the 
permanent  records  of  volcanic  action  must  be  of  two  kinds :  first 
and  most  obvious  are  the  piles  of  volcanic  materials  which  have 
been  spread  out  upon  the  surface  of  the  earth,  not  only  round 


Fig.  35. —  Cellular  Lava  with  a  few  of  the  cells  filled  up  with  infiltrated 
mineral   matter    (Amygdules). 

the  immediate  vents  of  eruption,  but  often  to  great  distances 
from  them;  secondly,  the  rents  and  other  openings  in  the  solid 
crust  of  the  earth  caused  by  the  volcanic  explosions,  and  some  of 
which  have  served  as  channels  by  which  the  volcanic  materials 
have  been  expelled  to  the  surface. 

VOLCANIC  PRODUCTS. —  We  shall  first  consider  those  materials 
which  are  erupted  from  volcanic  vents  and  are  heaped  up  on  the 
surface  as  volcanic  cones  or  spread  out  as  sheets.  They  may  be 
conveniently  divided  into  two  groups:  1st,  Lava,  and  2d,  Frag- 
mentary materials. 

(1)  Lava. —  Under  this  name  are  comprised  all  the  molten 
rocks  of  volcanoes.  These  rocks  present  many  varieties  in  com- 


VOLCANOES  AND  EARTHQUAKES 


101 


position  and  texture,  some  of  the  more  important  of  which  will 
be  described  in  Chapter  XI.  Most  of  them  are  crystalline  — 
that  is,  are  made  up  wholly  or  in  greater  part  of  crystals  of  two 
or  more  minerals  interlocked  and  felted  together  into  a  coherent 
mass.  Some  are  chiefly  composed  of  a  dark  brown  or  black 
glass,  while  others  consist  of  a  compact  stony  substance  with 
abundant  crystals  imbedded  in  it.  Probably  most  of  them 
when  in  completest  fusion  within  the  earth's  crust  existed  in 
the  condition  of  thoroughly  molten  glass,  the  transition  from 
that  state  to  a  stony  or  lithoid  one  being  due  to  a  process  of 
"devitrification"  (Ch.  XI)  consequent  on  cooling.  During  the 
process  some  of  the  component  ingredients  of  the  glass  crystallise 
out  as  separate  minerals,  and  this  crystallisation  sometimes  pro- 
ceeds so  far  as  to  use  up  all  the  glass  and  to  transform  it  into 
a  completely  crystalline  substance. 

In  many  cases  lavas  are  strikingly  cellular  —  that  is  to  say, 
they  contain  a  large  number  of  spherical  or  almond-shaped  cavi- 
ties somewhat  like  those  of  a  sponge  or  of  bread,  formed  by  the 
expansion  of  the  steam  absorbed  in  the  molten  rock  (Figs.  35 
and  36  and  Ch.  XI).  Lavas  vary  much  in  weight  and  in  colour. 
The  heavier  kinds  are  more  than  three  times  the  weight  of  water; 
or,  in  other  words,  they  have 
a  specific  gravity  ranging  up  to 
3.3;  and  are  commonly  dark 
gray  to  black.  The  lighter  va- 
rieties, on  the  other  hand,  are 
little  more  than  twice  the 
weight  of  water,  or  have  a  spe- 
cific gravity  which  may  be  as 
low  as  2.3,  while  their  colours 
are  usually  paler,  sometimes 
almost  white. 


Fig'  36'-  Section  ot  a 


When  lava  is  poured  out  at  the  surface  it  issues  at  a  white  heat 
—  that  is,  at  a  temperature  sometimes  above  that  of  melting 
copper,  or  more  than  2204°  Fahr.  ;  but  its  surface  rapidly  dark- 
ens, cools,  and  hardens  into  a  solid  crust  which  varies  in  aspect 
according  to  the  liquidity  of  the  mass.  Some  lavas  are  remark- 
ably fluid,  flowing  along  swiftly  like  melted  iron;  others  move 
sluggishly  in  a  stiff  viscous  stream.  In  many  pasty  lavas,  the 
surface  breaks  up  into  rough  cindery  blocks  or  scoriae  like  the 


102  GEOLOGY 

slags  of  a  foundry,  which  grind  upon  each  other  as  the  still 
molten  stream  underneath  creeps  forward  (Ch.  XI).  In  gen- 
eral, the  upper  part  of  a  lava-stream  is  more  cellular  than  the 
central  portions,  no  doubt  because  the  imprisoned  steam  can 
there  more  easily  expand.  The  bottom,  too,  is  often  rough  and 
slaggy,  as  the  lava  is  cooled  by  contact  with  the  ground,  and 
portions  of  the  chilled  bottom-crust  are  pushed  along  or  broken 
up  and  involved  in  the  still  fluid  portion  above. 

There  are  thus  three  more  or  less  well-defined  zones  in  a 
solidified  lava-current  —  a  cellular  or  slaggy  upper  part  (c  in 
Fig.  36),  a  more  solid  and  jointed  centre  (&),  embracing  usually 
by  much  the  largest  proportion  of  the  whole,  and  a  cellular  or 
slaggy  bottom  (a).  A  rock  presenting  these  characters  tells  its 


Fig.  37. —  Elongation  of  cells  in  direction  of  flow  of  a  lava-stream. 

story  of  volcanic  action  in  quite  unmistakable  language.  It  re- 
mains as  evidence  of  the  existence  of  some  neighbouring  volcanic 
vent,  now  perhaps  entirely  covered  up,  whence  it  flowed.  We 
may  even  be  able  to  detect  the  direction  in  which  the  lava 
moved.  The  cells  opened  by  the  segregation  and  expansion  of 
the  steam  entangled  in  the  interstices  of  a  mass  of  lava  which 
is  at  rest  are,  on  the  whole,  spherical.  But  if  the  rock  is  still 
moving,  the  cells  will  be  drawn  out  and  flattened  into  almond- 
shaped  (amygdaloidal)  vesicles,  with  their  flat  sides  parallel  to 
the  surface  of  the  lava,  and  their  longer  axes  ranged  in  one  gen- 
eral direction,  which  is  that  of  the  motion  of  the  molten  stream 
(Fig.  37). 

At  a  volcanic  vent,  the  mass  of  erupted  lava  is  generally 
thickest,  and  it  thins  away  as  its  successive  streams  terminate 


VOLCANOES  AND  EARTHQUAKES          103 

on  the  lower  grounds  surrounding  the  cone.  But  sometimes  a 
lava-current  may  flow  for  40  miles  or  more  from  its  source,  and 
may  here  and  there  attain  locally  a  great  thickness  by  rolling 
into  a  valley  and  filling  it  up,  as  has  been  witnessed  among  the 
Icelandic  eruptions.  As  a  rule,  where  ancient  lava-streams  are 
found  to  thicken  in  a  certain  direction,  we  may  reasonably  infer 
that  in  that  direction  lay  the  vent  from  which  they  flowed. 

Again,  sheets  of  lava  that  solidify  on  the  slopes  of  a  volcanic 
cone  are  inclined ;  they  may  congeal  on  declivities  of  as  much  as 
30°  or  40°.  If  a  series  of  ancient  lavas  were  observed  to  slope 
upward  to  a  common  centre,  we  might  search  there  for  some 
trace  of  the  funnel  from  which  they  were  discharged.  But,  of 
course,  in  proportion  to  their  antiquity,  lava-streams,  like  every 
other  kind  of  rock,  have  suffered  from  geological  revolutions, 
among  which  those  that  involve  upheaval  and  dislocation  are 
especially  important,  so  that  the  inclination  of  an  ancient  lava- 
bed  must  not  be  too  hastily  assumed  as  an  indication  of  the 
slope  of  the  cone  of  a  volcano.  It  must  be  taken  in  connection 
with  the  rest  of  the  evidence  supplied  by  the  whole  district. 

Where  lavas  reach  the  lower  grounds  beyond  the  foot  of  a 
volcanic  cone,  they  may  spread  out  in  wide  nearly  horizontal 
sheets.  As  current  succeeds  current,  the  original  features  of  the 
plain  may  be  entirely  buried  under  a  mass  of  lava  many  feet 
thick.  If  a  section  could  be  cut  through  such  an  accumulation, 
it  might  be  possible  to  determine  the  thickness  of  each  succes- 
sive lava-stream  by  means  of  the  slaggy  upper  and  lower  surfaces. 
Here  and  there,  too,  where  two  eruptions  were  separated  by  an 
interval  long  enough  to  allow  the  surface  of  the  older  mass  par- 
tially to  crumble  into  soil  and  support  some  vegetation,  the 
layer  of  burnt  soil  between  the  two  sheets  of  lava  would  remain 
as  a  witness  of  this  interval. 

In  other  instances,  we  can  understand  that  in  the  larger  hol- 
lows of  a  lava-plain,  ponds  or  lakes  might  gather,  on  the  floor 
of  which  there  might  be  deposited  layers  of  fine  silt  full  of  ter- 
restrial leaves,  insect  remains,  land  and  fresh-water  shells,  and 
other  organic  relics  of  a  land-surface.  If,  now,  a  lacustrine  ac- 
cumulation of  this  kind  were  to  be  buried  under  a  new  outburst 
of  lava,  it  would  be  sealed  up  and  might  preserve  its  record 
intact  for  vast  ages.  In  any  section  cut  through  such  a  series 
of  lava-beds  by  a  river  or  the  sea,  or  by  man,  the  layers  of  silt 


104  GEOLOGY 

with  their  organic  remains  intercalated  between  the  lava-streams 
would  prove  the  eruptions  to  have  taken  place  on  land,  and  to 
have  been  separated  by  a  long  interval,  during  which  a  lake  was 
formed  on  the  cold  and  decomposing  surface  of  the  earlier 
lava. 

The  conditions  under  which  the  volcanic  outbursts  occurred 
may  be  thus  inferred,  not  so  much  from  the  nature  of  the  vol- 
canic materials  themselves,  as  from  that  of  the  layers  of  sedi- 
ment that  may  happen  to  have  been  preserved  among  them. 
Seams  of  red  baked  soil,  with  charred  remains  of  terrestrial 
vegetation  interposed  between  the  upper  and  under  sides  of  suc- 
cessive lavas  would  point  to  subaerial  eruptions.  Bands  of  hard- 
ened clay  or  marl,  with  leaves  and  fresh-water  shells,  would  show 
that  the  lavas  had  invaded  a  lake.  Beds  of  limestone  or  other 
rock,  containing  corals,  sponges,  marine  shells,  and  other  traces 
of  the  life  of  the  sea,  would  demonstrate  that  the  eruptions  were 
submarine.  Examples  of  each  of  these  varieties  of  evidence 
occur  abundantly  among  the  old  volcanic  tracts  of  Britain. 

(2)  Fragmentary  Products. —  These  supply  some  of  the  most 
striking  proofs  of  volcanic  energy.  They  vary  in  size  from 
huge  blocks  of  stone,  weighing  many  tons,  down  to  the  finest 
dust.  The  coarsest  materials  naturally  accumulate  round  the 
vent,  while  the  finest  may  be  borne  away  by  wind  to  distances 
of  many  hundreds  of  miles.  On  the  volcano  itself,  the  stones, 
ashes,  and  dust  form  beds  of  coarse  and  fine  texture  which,  on 
the  outside  of  the  cone,  have  the  usual  slope  of  the  declivity.  By 
degrees,  they  become  more  or  less  consolidated,  and  are  then 
known  by  the  general  name  of  Tuffs. 

The  materials  composing  a  tuff  are  generally  derived  from 
lavas.  The  fine  dust,  discharged  from  a  large  volcano  in  such 
prodigious  quantities  as  to  make  the  sky  dark  ap  midnight  for 
days  together,  is  simply  lava  that  has  been  blown  into  this  finely 
divided  condition  by  the  explosion  of  the  vapours  and  gases 
which  exist  absorbed  in  it  while  still  deep  down  within  the 
earth's  crust.  The  cinder-like  fragments,  and  scoria?  or  slags, 
that  are  ejected  in  such  numbers  and  fall  back  into  the  crater 
and  upon  the  outer  slopes  of  the  cone  are  pieces  of  lava  frothed 
up  by  the  expansion  of  the  imprisoned  steam,  torn  off  from  the 
column  of  lava  in  the  vent,  and  shot  whirling  up  into  the  air. 
Large  blocks  of  lava,  or  of  the  rocks  through  which  the  volcanic 


VOLCANOES  AND  EARTHQUAKES  105 

funnel  has  been  opened,  are  often  broken  off  by  the  force  of  the 
explosions  and  discharged  with  the  other  volcanic  detritus  from 
the  vent.  These  materials,  descending  to  the  ground,  form  suc- 
cessive beds  that  vary  in  dimensions  according  to  the  vigour  of 
eruption  and  their  distance  from  the  vent.  Around  the  focus 
of  activity  there  may  be  thick  accumulations  of  blocks,  bombs, 
and  pieces  of  scoriae  mixed  with  fine  ashes  and  sand.  A  notable 
feature  is  the  generally  cellular  character  of  these  stones  —  a 
peculiarity  which  marks  them  as  made  of  truly  volcanic  materials. 
An  examination  of  the  finest  dust  likewise  discloses  the  presence 
of  the  glass  and  crystals  that  constitute  the  lava  from  the  ex- 
plosion of  which  the  dust  was  derived. 

From  the  wide  area  over  which  the  fragmentary  materials 
ejected  by  volcanoes  are  dispersed  in  the  atmosphere  before  they 
fall  to  the  earth,  they  are  more  likely  than  lavas,  to  be  preserved 
among  contemporaneous  sedimentary  accumulations.  They 
often  descend  upon  lakes,  and  must  there  be  interstratified  with 
the  mud,  marl,  or  other  deposit  in  progress  at  the  time.  They 
are  also  widely  diffused  over  the  sea-floor.  Eecent  dredgings  of 
the  ocean-basins  have  shown  that  traces  of  fine  volcanic  detritus 
may  be  detected  even  at  remote  distances  from  land.  As  active 
volcanoes  almost  always  rise  near  the  sea,  as  the  oceans  are 
dotted  over  with  volcanic  islands,  and  as,  doubtless,  many  erup- 
tions take  place  on  the  sea-bottom,  there  are  obvious  reasons  why 
volcanic  particles  should  be  universally  diffused  over  the  sea-bot- 
tom. Geologists  can  also  understand  why,  in  the  records  of 
volcanic  history  in  bygone  ages,  so  large  a  proportion  of  the 
evidence  should  be  of  submarine  eruptions. 

Beds  of  tuff  often  contain  traces  of  the  plants  or  animals  that 
lived  on  the  surfaces  on  which  the  volcanic  materials  fell  — 
sometimes  remains  of  terrestrial  or  lacustrine  vegetation  and 
animals ;  but  in  the  great  majority  of  instances,  shells  and  other 
relics  of  the  inhabitants  of  the  sea. 

In  a  series  of  layers  of  tuff  round  a  volcanic  orifice,  the 
memorials  of  the  earliest  discharges  are  of  course  preserved  in 
the  layers  at  the  bottom.  Accordingly,  in  such  situation?, 
abundant  fragments  of  the  rocks  of  the  surrounding  country 
may  be  noticed.  We  could  hardly  ask  for  more  convincing  evi- 
dence of  the  blowing  out  of  the  vent  and  the  ejection  of  the  rock- 


106 


GEOLOGY 


fragments  from'  it,  before  the  volcano  began  to  discharge  only 
volcanic  materials. 

Large  blocks  of  lava  ejected  obliquely  from  the  crater  may  fall 
beyond  the  limits  of  the  cone.  If  a  block  thus  discharged  should 
fall  into  a  lake-basin,  it  would  be  covered  up  in  the  silt  ac- 
cumulating there,  and  might  be  the  only  remaining  record  of 
the  eruption  to  which  it  belonged.  In  after  times,  were  the 
lake-floor  laid  dry,  the  stone  might  be  found  in  its  place,  the 

layers  of  sediment  into  which 
it  fell  pressed  down  by  the 
force  with  which  it  landed  on 
them  and  by  its  weight,  the 
later  layers  mounting  over 
and  covering  it.  Examples 
of  this  kind  of  evidence  may 


3  •-  — 


Fig.  38. —  Volcanic  block  ejected 
during  the  deposition  of  strata 
in  water. 


be  gathered  in  many  old  vol- 
canic districts.  One  taken 
from  the  coast  of  Fife  is  given  in  Fig.  38.  The  lowest 
bed  there  shown  (1)  is  a  brown  shaly  fire-clay,  about  5  inches 
thick,  which  was  once  a  vegetable  soil,  for  abundant  rootlets  can 
be  seen  branching  through  it.  It  is  overlain  by  a  seam  of  coal 
(2),  5  or  6  inches  thick,  representing  the  dense  growth  of 
vegetation  that  flourished  upon  the  soil;  the  next  layer  (3)  is  a 
green  crumbling  fire-clay  about  a  foot  thick,  covered  by  a  dark 
shale  with  remains  of  plants  (4).  The  feature  of  special  in- 
terest in  this  section  is  the  angular  block  of  lava  (diabase) 
weighing  about  six  or  eight  pounds,  which  is  stuck  vertically 
in  bed  No.  3.  There  can  be  no  doubt  that  this  was  ejected  by 
an  explosion,  of  which  there  is  here  no  other  record.  It  proba- 
bly descended  with  some  force,  for,  as  shown  in  the  drawing,  the 
lower  layers  of  the  fire-clay  are  pressed  down  by  it,  and  the 
coal  itself  is  compressed.  We  see  that  the  stone  fell  before  the 
upper  half  of  the  fire-clay  was  formed,  for  the  layers  of  that  part 
of  the  bed  are  heaped  around  the  stone  and  finally  spread  over 
it.  There  are  layers  of  lava  and  tuff  above  and  below  the  strata 
depicted  in  this  section,  so  that  there  is  abundant  other  evidence 
of  preceding  and  subsequent  volcanic  action  here  (see  Fig.  109). 
As  the  fine  volcanic  dust  may  be  transported  by  wind  for 
hundreds  of  miles  before  reaching  the  surface  of  the  earth,  its 
presence  does  not  necessarily  show  that  a  volcano  existed  in  the 


VOLCANOES  AND  EARTHQUAKES          107 

neighbourhood  in  which  it  fell.  The  fine  ashes  from  the  Ice- 
landic volcanoes,  for  example,  have  been  found  abundantly  even 
as  far  as  Sweden  and  the  Orkney  Islands.  But  where  the  frag- 
mentary materials  are  of  coarse  grain,  and  more  especially  where 
they  contain  large  slags,  scoriaB,  and  bombs,  and  where  they  are 
interstratified  with  sheets  of  lava,  they  unquestionably  indicate 
the  proximity  of  some  volcanic  vent  from  which  the  whole  pro- 
ceeded. 

VOLCANIC  VENTS  AND  FISSURES. —  The  various  materials 
ejected  from  a  volcano  to  the  surface  may  conceivably  be  in 
course  of  time  entirely  swept  away.  Nevertheless,  though  every 
sheet  of  lava  and  every  bed  of  tuff  is  removed,  there  will  still  re- 
main the  filled  up  vent,  funnel,  or  fissure,  up  which  these  ma- 
terials rose,  and  out  of  which  they  were  ejected.  This  open- 
ing in  the  solid  crust  of  the  earth  must  evidently  be  one  of  the 
most  durable,  as  it  is  certainly  one  of  the  fundamental  features 
in  a  volcano.  Let  us  first  consider  the  vent  of  an  ordinary  vol- 
cano. 

In  many  instances,  there  is  reason  to  believe  that  volcanic 
vents  are  opened  along  lines  of  fracture  in  the  earth's  crust. 
This  seems  especially  to  be  the  case  where  a  group  of  volcanoes 
runs  along  one  definite  line,  as  is  represented  in  Fig.  39.  Along 


Fig.  39. —  Volcanoes  on  lines  of  fissure. 

such  a  fissure,  either  of  older  date  or  due  to  the  energy  of  the 
volcanic  explosions  themselves,  there  must  be  weaker  places 
where  the  overlying  mass  is  less  able  to  bear  the  strain  of  the 
pent-up  vapours  underneath.  At  these  places,  after  successive 
shocks,  openings  may  at  length  be  made  to  the  surface,  whence 
the  lavas  and  ashes  will  be  emitted,  and  each  such  opening  will 
be  marked  by  a  cone  of  the  erupted  material. 

But  in  innumerable  examples,  it  is  found  that  a  fissure  is  not 
necessary  for  the  formation  of  a  volcano.     The  effect  of  the 


108  GEOLOGY 

volcanic  explosions  is  such  as  to  drill  a  pipe  or  funnel  even 
through  the  solid  unfractured  crust  of  the  earth.  The  volcanic 
energy,  so  far  from  requiring  a  line  of  fracture  for  its  assistance, 
seems  often  to  have  avoided  making  use  of  such  a  line,  even  when 
it  existed.  In  the  volcanic  plateaux  of  Utah,  which  are  dotted 
over  with  little  volcanic  cones,  and  are  also  traversed  by  great 
dislocations,  it  is  noticeable  that  the  vents  are  not  clustered 
along  the  lines  of  fracture.  In  the  same  region,  and  also  along 
the  courses  of  the  Ehine  and  Moselle,  volcanic  vents  have  been 
opened  near  the  brink  of  deep  ravines  rather  than  at  the  bottom. 
These  are  features  of  volcanic  action,  for  which  no  satisfactory 
explanation  has  yet  been  found. 

The  crater  of  an  active  volcano  is  a  hideous  yawning  cauldron, 
with  rough  red  and  black  walls,  at  the  bottom  of  which  lie  steam- 
ing pools  of  molten  lava.  Every  now  and  then  a  sharp  explosion 
tears  the  lava  open,  sending  up  a  shower  of  glowing  fragments 
and  hot  ashes.  These  pools  of  liquid  lava  lie  evidently  on  the 
top  of  a  column  of  melted  rock  which  descends  in  the  volcanic 
chimney  to  an  unknown  depth  into  the  earth's  interior.  If  the 
volcano  were  to  become  extinct,  this  lava  column  would  cool  and 
solidify,  and  even  after  the  entire  destruction  and  removal  of  the 
cone  and  crater,  would  remain  as  a  stump  to  tell  where  the  site 
of  the  volcano  had  been.  Layer  after  layer  might  be  stripped  off 
the  surface  of  the  land;  hundreds  or  thousands  of  feet  of  rock 
might  in  this  manner  be  removed,  yet,  so  far  as  we  know,  the 
stump  of  the  volcano  would  still  be  there.  No  probable  amount 
of  waste  of  the  surface  of  the  earth's  crust  could  remove  a 
vertical  column  of  rock  which  descends  to  an  unknown  depth 
into  the  interior.  The  site  of  a  volcanic  vent  can  never  be  effaced 
except  by  being  buried  under  masses  of  younger  rock. 

Volcanic  vents,  affording  as  they  do  so  durable  a  testimony  to 
volcanic  action,  deserve  careful  attention.  At  an  active  volcano, 
or  even  at  one  which,  though  extinct,  still  retains  its  cone  of 
erupted  materials,  we  cannot,  of  course,  learn  much  regarding 
the  shape  and  size  of  the  funnel,  for  only  the  crater,  and  at  most 
merely  the  upper  part  of  the  vent,  are  accessible.  But  among 
volcanic  tracts  of  older  date,  where  the  cones  have  been  destroyed, 
and  where  the  filled-up  funnels  are  laid  bare,  the  subterranean 
architecture  of  volcanoes  is  revealed  to  us.  At  such  places  we 
are  allowed,  as  it  were,  to  descend  the  chimney  of  a  volcano,  and 


VOLCANOES  AND  EARTHQUAKES  109 

to  make  observations  altogether  impossible  at  a  modern  volcanic 
cone. 

From  observations  made  at  such  favourable  localities,  it  has 
been  ascertained  that  the  funnels  of  volcanoes  are  in  general 
rudely  circular  or  elliptical,  though  liable  to  many  modifications 
of  outline.  They  vary  indefinitely  in  diameter,  according  to  the 
vigour  of  the  volcanic  outbursts  that  produced  them.  The 
smaller  vents  are  not  more  than  a  few  yards  in  width ;  but  those 
of  the  larger  volcanoes  which,  as  in  Sumatra  and  Java,  have 
sometimes  craters  comprising  an  area  of  40  square  miles,  must 
have  enormously  larger  funnels. 

The  materials  that  fill  up  a  vent  are  sometimes  only  fragments 
of  the  surrounding  rocks.  In  such  cases,  we  may  suppose  that 
when  the  volcanic  explosions  had  spent  their  force  and  had  blown 
out  an  opening  to  the  surface  of  the  ground,  they  were  not 
succeeded  by  the  uprise  of  any  solid  volcanic  materials ;  that,  in 
short,  only  the  first  stage  in  the  establishment  of  a  volcano 


Fig.  40. —  Outline  of  a  Volcanic  Neck. 

was  reached,  when,  owing  to  some  failure  of  the  subterranean 
energy  at  the  place,  the  operations  came  to  an  end.  But  though 
the  upper  part  of  the  vent  might  remain  open,  surrounded  with  a 
crater  formed  of  the  fragments  into  which  the  rocks  were  blown 
by  the  explosions,  the  lower  parts  would  undoubtedly  be  filled  up 
by  the  fall  of  fragments  back  again  into  the  vent.  And  if  all 
the  material  ejected  to  the  surface  were  removed,  the  top  of  this 
column  of  fragmentary  materials  would  remain  as  an  unmistak- 
able evidence  of  the  explosions  that  had  originated  it. 

But  in  the  vast  majority  of  cases,  the  operations  at  a  volcanic 
vent  do  not  end  with  the  first  explosions.  Clouds  of  ashes  and 
stones  are  ejected,  and  streams  of  molten  lava  are  poured  forth. 
In  some  instances,  the  chimney  may  be  finally  choked  with  vol- 
canic blocks,  scoriae,  cinders,  and  ashes,  in  others  with  consoli- 
dated lava.  Examples  of  both  kinds  of  infilling  are  found,  and 


110 


GEOLOGY 


also  others  where  the  two  forms  of  volcanic  material  occur  to- 
gether in  the  same  vent. 

A  volcanic  chimney  filled  up  in  this  way  with  volcanic  ma- 
terials and  exposed  by  the  removal  of  the  lava  or  ashes  thrown 
out  to  the  surface  is  known  as  a  Neck  (see  Figs.  40,  41,  42,  111, 
and  Ch.  XIV).  As  these  materials  are  usually  harder  and  more 
durable  than  the  surrounding  rocks,  they  project  above  the  gen- 
eral surface  of  the  ground.  The  stump  of  the  volcano  is  left  as 
a  hill,  the  form  and  prominence  of  which  will  chiefly  depend 
upon  the  nature  of  the  material;  hard  tough  lava  will  rise 
abruptly,  as  a  crag  or  hill,  above  the  surrounding  country,  while 
consolidated  ashes,  scoriae,  and  other  fragmentary  stuff  will  give 
a  smoother  and  less  marked  outline. 

These  features  will  be  best  understood  from  a  series  of  dia- 
grams. We  may  take,  by  way  of  illustration,  a  neck  composed 
mainly  of  fragmentary  ejections,  but  with  a  plug  of  lava  reach- 
ing its  summit.  The  usual  outlines  of  such  a  neck  are  repre- 
sented in  Fig.  40.  There  is  nothing  in  the  general  form  of  this 

hill  to  suggest  a  volcanic  origin; 
yet,  if  we  examine  its  structure 
and  that  of  the  ground  around 
it,  we  may  find  them  to  be  as 
represented  in  Fig.  41,  where 
the  surrounding  rocks  are  sup- 
posed to  consist  of  various  sand- 
stones, clays,  limestones,  and 
other  sedimentary  deposits  (a), 
through  which  the  volcanic  vent 
(b,  c)  has  been  drilled.  The  neck  is  represented  as  elliptical 
in  cross  section,  composed  mainly  of  consolidated  volcanic  ashes 
and  blocks  (&),  but  with  a  mass  of  lava  (c)  in  the  centre.  The 
structure  of  the  hill  is  explained  in  the  vertical  section,  Fig  42 
(see  also  Fig.  111).  We  there  see  that  the  vent  has  been  blown 
through  the  surrounding  strata  (a,  a),  and  has  been  filled  up 
mainly  with  fragmentary  materials  (&,  b) ;  but  that  through  its 
centre  there  has  risen  a  column  or  plug  of  lava  (c),  which  not 
improbably  marks  the  last  effort  of  the  volcano  to  force  solid 
ejections  to  the  surface.  The  line  s,  s  indicates  the  present  sur- 
face of  the  ground,  after  the  prolonged  waste  during  which  all 
the  volcanic  cone  has  been  removed.  But  we  can  in  imagina- 


Fig.  41. —  Ground-plan  of  the 
structure  of  the  Neck  shown 
in  Fig.  40. 


VOLCANOES  AND  EARTHQUAKES 


111 


tion  restore  the  original  surface,  which  may  have  been  some- 
what as  shown  by  the  dotted  lines,  the  position  of  the  crater  be- 
ing indicated  at  e,  and  its  crest  on  either  side  at  d,  d.  No  trace 
is  here  left  of  the  original  volcanic  cone.  The  present  form  of 
the  ground  is  due  to  denudation,  which  has  left  the  more  durable 
volcanic  rocks  projecting  above  the  surrounding  strata.  The 
continued  progress  of  superficial  degradation  will  remove  still 
more  of  the  neck,. but  the  downward  continuation  of  the  volcanic 
column  must  always  remain,  and  will  probably  always  project  as 
a  hill.  A  volcanic  neck  is  thus  one  of  the  most  enduring  and 
unmistakable  evidences  of  the  site  of  a  volcano. 

Besides  vents  or  funnels,  other  openings  are  made  by  vol- 
canic explosions  in  the  crust,  which  serve  as  receptacles  of  lava 


Fig.  42. —  Section  through  the  same  Neck  as  in  Figs.  40  and  41. 

and  ashes,  and  remain  as  durable  memorials  of  volcanic  action. 
Of  these  the  most  important  are  Fissures,  which  are  formed  in 
large  numbers  in  and  around  a  volcanic  cone,  but  which  may 
also  arise  at  a  distance  from  any  actual  volcano.  During  the 
convulsions  of  an  eruption,  the  cone  and  the  surrounding 
country  are  sometimes  split  by  lines  of  fissure,  which  tend  to 
radiate  from  the  centre  of  disturbance,  somewhat  as  cracks  do  in 
a  pane  of  glass  through  which  a  stone  is  thrown.  Sometimes 
the  two  sides  of  a  fissure  close  together  again,  leaving  no  super- 
ficial trace  of  the  dislocation.  More  frequently  steam  and  vari- 
ous volcanic  vapours  escape  from  the  chasm,  and  may  deposit 
along  the  walls  sublimates  of  different  minerals,  such  as  common 
salt,  chloride  of  iron,  specular  iron,  sulphur,  and  sal-ammoniac. 
These  deposited  substances  may  even  continue  to  grow  there 
until  they  entirely  fill  up  the  space  between.  In  such  cases,  the 
line  of  fissure  is  marked  by  a  vertical  or  steeply  inclined  band 


112 


GEOLOGY 


of  minerals  interposed  between  the  ends  of  the  rocks  that  have 
been  ruptured  and  separated.  But  in  most  instances,  the  open- 
ing is  filled  up  by  the  rise  of  lava  from  below.  At  night,  the 
vents  opened  on  the  outside  of  an  active  volcano  may  be  traced 
from  afar  by  the  glow  of  the  white-hot  lava  that  rises  in  them 
to  within  a  short  distance  from  the  surface.  When  the  lava 
cools  and  solidifies  in  these  fissures,  it  forms  wall-like  masses, 
known  as  Dykes  (Fig.  43).  Inside  many  volcanic  craters,  the 
walls  are  traversed  with  dykes  which,  though  on  the  whole  tend- 


Fig.  43. —  Volcanic  dykes  rising  through  the  bedded  tuff  of  a  crater. 

ing  to  keep  a  vertical  direction,  may  curve  about  irregularly  ac- 
cording to  the  form  of  the  vents  into  which  the  lava  rose.  Like 
the  necks  above  described,  dykes  form  enduring  records  of  vol- 
canic action.  The  superficial  cones  and  craters  may  disappear, 
but  the  subterranean  lava-filled  fissures  will  still  remain  as 
records  of  volcanic  action. 

In  some  volcanic  regions,  where  enormous  floods  of  lava  have 
been  poured  forth,  no  great  central  cones  have  existed.  Such 
regions  extend  as  vast  black  plains  of  naked  rock,  mottled  with 
shifting  sand-hills,  or  as  undulating  tablelands  carved  by  run- 
ning water  into  valleys  and  ravines,  between  which  the  successive 


VOLCANOES  AND  EARTHQUAKES          113 

sheets  of  lava  are  exposed  in  terraced  hills.  Beyond  the  limits 
over  which  the  lava-sheets  are  spread,  dykes  of  the  same  kinds 
of  lava  rise  in  abundance  to  the  surface.  There  can  be  no  doubt 
that  the  dykes  do  not  terminate  at  the  edge  of  the  lava-fields, 
but  pass  underneath  them.  Indeed,  as  they  increase  in  number 
in  that  direction,  they  are  probably  more  abundant  underneath 
the  lava  than  outside  of  the  lava-fields.  Sometimes  sections  are 
exposed  showing  how,  after  rising  in  a  fissure,  the  lava  has  spread 
out  on  either  side  as  a  sheet.  In  these  vast  lava-plateaux  or 
deserts,  the  molten  rock,  instead  of  issuing  from  one  main  central 
Etna  or  Vesuvius,  appears  to  have  risen  in  thousands  of  fissures 
opened  in  the  shattered  crust,  and  to  have  welled  forth  from 
numerous  vents  on  these  fissures,  spreading  out  sheet  after  sheet 
till,  like  a  rising  lake,  it  has  not  only  overflowed  the  lower 
grounds,  but  even  buried  all  the  minor  hills.  Such  appears  to 
have  been  the  history  of  vast  tracts  in  Western  North  America. 
The  area  which  has  there  been  flooded  with  lava  has  been  esti- 
mated to  be  larger  than  that  of  France  and  Great  Britain  to- 
gether, and  the  depth  of  the  total  mass  of  lava  erupted  reaches 
in  some  places  as  much  as  3700  feet.  Some  rivers  have  cut 
gorges  in  this  plain  of  lava,  laying  bare  its  component  rocks  to 
a  depth  of  700  feet  or  more.  Along  the  walls  of  these  ravines 
we  see  that  the  lava  is  arranged  in  parallel  beds  or  sheets  often 
not  more  than  10  or  20  feet  thick,  each  of  which,  of  course,  repre- 
sents a  separate  outpouring  of  molten  rock. 

Except  where  such  deep  sections  have  been  cut  through  them 
by  rivers,  recent  lava-floods  can  only  be  examined  along  their 
surface,  and  we  are  consequently  left  chiefly  to  inference  re- 
garding their  probable  connection  with  fissures  and  dykes  under- 
neath. But  in  various  parts  of  the  world,  lava-plains  of  much 
older  date  have  been  so  deeply  eroded  as  to  expose  not  only  the 
successive  sheets  of  lava  but  the  floor  over  which  they  were 
poured,  and  the  abundant  dykes  which  no  doubt  served  as  the 
channels  wherein  the  lava  rose  towards  the  surface,  till  it  could 
escape  at  the  lowest  levels,  or  at  weaker  or  wider  parts  of  the 
fissures.  In  Western  Europe  important  examples  of  this  struc- 
ture occur,  from  the  north  of  Ireland  through  the  Inner  Hebrides 
and  the  Faroe  Islands  to  Iceland.  This  volcanic  belt  presents  a 
succession  of  lava-fields  which  even  yet,  in  spite  of  enormous 
waste,  are  in  some  places  more  than  3000  feet  thick.  The  sheets 


114  GEOLOGY 

of  lava  are  nearly  flat,  and  rise  in  terraces  one  over  another  into 
green  grassy  hills,  or  into  the  dark  fronts  of  lofty  sea-washed 
precipices.  Where  this  thick  cake  of  lava  has  been  stripped  off 
during  the  degradation  of  the  land,  thousands  of  dykes  are  ex- 
posed, and  many  of  these  traverse  at  least  the  lower  parts  of  the 
sheets  of  lava.  They  form,  as  it  were,  the  subterranean  roots 
of  which  these  sheets  were  the  subaerial  branches;  and  even 
where  the  whole  of  the  material  that  reached  the  surface,  more 
than  3000  feet  thick,  has  been  worn  away,  the  dykes  still  remain 
as  evidence  of  the  reality  and  vigour  of  the  volcanic  forces. 

EARTHQUAKES. 

The  rise  of  hot  springs  and  the  explosions  of  volcanoes  furnish 
impressive  testimony  to  the  internal  heat  of  our  planet ;  but  they 
are  by  no  means  the  only  proofs  that  the  pent-up  energy  of  the 
interior  of  the  globe  reacts  upon  the  outer  surface.  By  means 
of  delicate  instruments,  it  can  be  shown  that  the  ground  beneath 
our  feet  is  subject  to  continual  tremors  which  are  too  feeble 
to  be  perceived  by  the  unaided  senses.  From  these  minuter 
vibrations,  movements  of  increasing  intensity  can  be  detected  up 
to  the  calamitous  earthquake,  whereby  a  country  is  shaken  to  its 
foundations,  and  thousands  of  human  lives,  together  with  much 
valuable  property,  are  destroyed.  We  do  not  yet  know  by  what 
different  causes  these  various  disturbances  are  produced.  Some 
of  the  fainter  tremors  may  arise  from  such  influences  as  changes 
of  temperature  and  atmospheric  pressure,  and  the  rise  and  fall 
of  the  tides.  But  the  more  violent  must  be  assigned  to  causes 
working  within  the  earth  itself.  The  collapse  of  the  roofs  of 
underground  caverns,  the  sudden  condensation  of  steam  or  ex- 
plosion of  volcanic  vapours,  the  snap  of  rocks  that  can  no  longer 
resist  the  strain  to  which,  by  the  cooling  and  consequent  con- 
traction of  the  inner  hot  nucleus,  they  have  been  subjected  with- 
in the  earth's  crust  —  these  and  other  influences  may  at  different 
times  come  into  play  to  determine  convulsive  earthquake  shocks. 
Without,  however,  entering  into  the  difficult  question  of  the 
causes  of  the  movements,  we  may  inquire  into  their  effects  in  so 
far  as  these  register  their  passing  in  the  annals  of  geological  his- 
tory. 

Their  awful  suddenness  and  devastation  have  invested  earth- 
quakes with  a  high  importance  in  the  popular  estimate  of  the 


VOLCANOES  AND  EARTHQUAKES          115 

forces  by  which  the  surface  of  the  globe  is  modified.  Yet  if  we 
judge  of  them  by  their  permanent  effects,  we  must  give  them  a 
comparatively  subordinate  place  among  these  forces.  After 
some  of  the  most  destructive  earthquakes  recorded  in  human 
history,  hardly  any  trace  of  the  calamity  is  to  be  seen,  save  in 
shattered  and  prostrate  houses.  But  when  these  buildings  have 
been  repaired  or  rebuilt,  no  one  visiting  the  ground  might  be 
able  to  detect  any  trace  of  the  earthquake  that  shattered  or 
overthrew  them. 

Yet  severe  earthquakes  do  not  pass  without  their  self -written 
chronicle  which,  though  often  evanescent  on  the  face  of  nature, 
is  at  the  time  conspicuous  enough.  Landslips  are  caused,  large 
masses  of  earth  and  blocks  of  rock  being  shaken  down  from 
higher  to  lower  levels;  the  ground  is  rent,  and  the  fissures  are 
sometimes  subsequently  widened  and  deepened  by  rain  and  run- 
nels into  ravines.  But  more  important  are  the  marked  changes 
of  level  that  occasionally  accompany  earthquake-shocks.  In 
some  cases,  the  ground  is  raised  for  several  feet,  so  that  along 
maritime  tracts  there  is  a  gain  of  land  from  the  sea;  in  others, 
the  ground  sinks,  and  the  sea  flows  in  upon  the  land.  Yet  it  is 
evident  that  unless  these  changes  are  actually  witnessed  as  the 
accompaniments  of  the  earthquakes,  they  may  take  place  without 
retaining  any  evidence  that  they  were  produced  by  such  a  cause. 
The  convulsion  of  an  earthquake,  notwithstanding  the  havoc  it 
may  bring  to  the  human  population  of  a  country,  does  not  always 
record  itself  in  distinctive  and  enduring  characters  in  geological 
history.  Some  of  its  most  noticeable  effects  also  are  not  due 
directly  to  its  own  action,  but  to  the  operations  of  the  waters  of 
the  land  and  of  the  sea  which,  when  disturbed  by  the  shock,  not 
infrequently  acquire  increased  vigour  in  their  own  peculiar  forms 
of  activity.  The  great  waves  set  in  motion  by  an  earthquake  roll 
over  the  low  lands  bordering  the  sea,  and  may  cause  vastly  more 
destruction  than  is  done  by  the  mere  shock  of  the  earthquake 
itself. 

UPHEAVAL  ANB  SUBSIDENCE. 

It  is  perhaps  not  so  much  by  earthquakes,  as  by  quiet,  hardly 
perceptible  movements,  that  the  relative  positions  of  sea  and  land 
are  undergoing  change  at  the  present  time.  In  some  parts  of 


116  GEOLOGY 

the  world  the  land  is  gradually  rising,  in  others  it  is  slowly  sink- 
ing. Proofs  of  elevation  are  supplied  by  lines  of  barnacles  or 
rock-boring  shells,  now  standing  above  the  reach  of  the  highest 
tides;  by  caves  that  have  obviously  been  scooped  out  by  the  sea, 
but  now  stand  at  a  higher  level  than  the  waves  can  reach ;  and  by 
deposits  of  sand,  gravel,  and  shells  which  were  evidently  accumu- 
lated on  a  beach,  but  which  now  rise  above  the  level  where 
similar  materials  are  now  being  accumulated  (Raised  Beaches). 
Evidences  of  subsidence  are  furnished  by  traces  of  old  land- 
surfaces  —  trees  with  roots  in  situ,  and  beds  of  peat,  lying  below 
the  limits  of  the  tides  (Submerged  Forests).  But  it  must  be 
more  difficult  to  prove  subsidence  than  elevation,  for  as  the 
land  sinks,  its  surface  is  carried  below  the  waves,  which  soon 
efface  the  evidence  of  terrestrial  characters. 

The  time  within  which  man  has  been  observing  and  recording 
the  changes  of  the  earth's  surface  forms  but  an  insignificant 
fraction  of  the  ages  through  which  geological  history  has  been  in 
progress.  We  cannot  suppose  that  during  this  brief  period  he 
has  had  experience  of  every  kind  of  geological  process  by  which 
the  outlines  of  land  and  sea.  are  modified.  There  may  be  great 
terrestrial  revolutions  which  happen  so  rarely  that  none  has 
occurred  since  man  began  to  take  note  of  such  things.  Among 
these  revolutions,  of  which  he  has  had  as  yet  no  experience,  the 
most  gigantic  is  the  formation  of  a  mountain-chain.  That  the 
various  mountain-chains  of  the  globe  are  of  very  different  ages, 
and  that  some  of  the  most  gigantic  of  them  are,  compared  with 
others,  of  recent  date,  are  facts  in  the  history  of  the  globe  which 
will  be  more  fully  referred'  to  in  later  pages ;  but  so  far  as  human 
history  or  tradition  goes,  man  has  never  witnessed  the  uprise  of  a 
range  of  mountains.  The  crust  of  the  earth  has  been  folded  and 
crumpled  on  the  most  colossal  scale,  some  parts  having  been 
pushed  for  miles  away  from  their  original  position;  it  has  been 
rent  by  profound  fissures,  on  each  side  of  which  the  rocks  have 
been  displaced  for  many  thousand  feet;  and  it  has  been  so 
broken,  crushed,  and  sheared,  that  its  component  rocks  have  in 
some  places  assumed  a  structure  entirely  different  from  what  they 
originally  possessed.  But  of  all  these  colossal  mutations  there  is 
no  human  experience.  We  are  driven  to  reason  regarding  them 
from  the  record  of  them  preserved  among  the  rocks,  and  from 
the  analogies  that  can  be  suggested  by  experiments  devised  to 


VOLCANOES  AND  EARTHQUAKES          117 

imitate  as  far  as  possible  the  processes  of  nature.  To  this  sub- 
ject we  shall  return  in  Chapter  XIII. 

SUMMARY. —  The  enduring  records  left  by  volcanoes,  whence 
their  former  existence  in  almost  all  regions  of  the  world  may  be 
demonstrated,  are  to  be  sought  partly  in  the  materials  which 
they  have  brought  up  to  the  surface,  and  partly  in  the  vents  and 
fissures  by  which  they  have  discharged  these  materials.  Of  the 
former  kind  of  evidence  lava  furnishes  a  conspicuous  example; 
its  internal  crystalline  or  glassy  structure,  its  steam-cavities,  and 
the  cellular  slaggy  upper  and  under  parts  of  the  sheets  in  which 
it  lies,  are  all  proofs  of  its  former  molten  condition.  A  succes- 
sion of  lava-beds,  piled  one  above  another,  marks  a  series  of  vol- 
canic eruptions,  and  the  nature  of  the  layers  of  non-volcanic  ma- 
terial intercalated  between  them  may  indicate  the  conditions 
under  which  the  eruptions  took  place,  whether  on  land,  in  lakes, 
or  in  the  sea.  The  fragmentary  products  consolidated  into  beds 
of  tuff  are  likewise  characteristic  of  volcanoes;  they  consist 
mainly  of  lava-dust  with  cindery  scoriae,  slags,  and  blocks ;  they 
accumulate  most  deeply  and  in  coarsest  material  at  and  im- 
mediately around  the  volcanic  vents,  but  their  finer  particles 
may  be  carried  to  enormous  distances ;  they  are  especially  liable 
to  be  intercalated  with  contemporaneous  sedimentary  deposits 
in  lakes  and  on  the  sea-floor. 

The  vents,  through  which  lava  and  ashes  are  ejected  to  the 
surface  form  the  most  permanent  record,  of  volcanoes,  for,  being 
filled  up  with  volcanic  rocks  to  unknown  depths,  they  cannot  be 
destroyed  by  the  mere  denudation  of  the  surface,  and  can  only 
disappear  by  being  buried  under  later  accumulations.  Such 
"necks"  consist  sometimes  of  lava,  sometimes  of  consolidated 
volcanic  debris,  or  of  both  kinds  of  material  together,  and  remain 
as  the  stumps  of  volcanoes,  where  every  other  trace  of  volcanic 
action  may  have  passed  away.  Not  less  enduring  are  the  dykes 
or  wall-like  masses  of  lava  which  have  risen  and  solidified  in  open 
fissures.  Enormous  sheets  of  lava  appear  to  have  flowed  out 
from  such  fissures  in  regions  where  the  volcanic  energy  never 
produced  any  great  central  cone. 

Earthquakes  do  not  impress  their  mark  upon  geological  his- 
tory so  indelibly  as  might  be  supposed.  In  spite  of  the  destruc- 
tion which  they  cause  to  human  life  and  property,  it  is  by  such 
direct  changes  as  landslips,  rents  of  the  ground,  and  the  up- 


118  GEOLOGY 

heaval  or  depression  of  land,  and  by  such  indirect  changes  as 
may  be  produced  by  derangements  of  rivers,  lakes,  and  the  sea, 
that  earthquakes  leave  their  chief  record  behind  them. 

Some  of  the  most  important  changes  of  level  now  going  on  are 
effected  quietly  and  almost  imperceptibly,  some  regions  being 
slowly  elevated,  and  others  gradually  depressed.  But  the  time 
within  which  man  has  been  an  observer  and  recorder  of  nature 
is  too  brief  to  have  supplied  him  with  experience  of  all  the  ways 
in  which  the  internal  energy  of  the  globe  affects  its  surface.  In 
particular,  he  has  never  witnessed  the  production  of  a  mountain- 
chain,  nor  any  of  the  plications,  fractures,  and  displacements 
which  the  crust  of  the  earth  has  undergone.  Eegarding  these 
revolutions  we  can  only  reason  from  the  records  of  them  in  the 
rocks,  and  from  such  laboratory  experiments  as  may  seem  most 
closely  to  imitate  the  processes  of  nature  that  were  concerned  in 
their  production. 


ELEMENTS  AND  MINERALS  119 


PAET  II. 

BOOKS,  AND  HOW  THEY  TELL  THE  HISTOKY  OF 
THE  EARTH. 

CHAPTER  X. 

ELEMENTS    AND    MINERALS. 

IN"  the  foregoing  Part  of  this  volume  we  have  been  engaged  in 
considering  the  working  of  various  processes  by  which  the 
surface  of  the  earth  is  modified  at  the  present  time,  and 
some  of  the  more  striking  ways  in  which  the  record  of  these 
changes  is  preserved.  We  have  seen  that,  on  the  whole,  it  is  by 
deposits  of  some  kind,  laid  down  in  situations  where  they  can 
escape  destruction,  that  the  story  of  geological  revolution  is 
chronicled.  In  one  place  it  is  the  stalagmite  of  a  cavern,  in 
another  the  silt  of  a  lake-bottom,  in  a  third  the  sand  and  mud 
of  the  sea-floor,  in  a  fourth  the  lava  and  ashes  of  a  volcano.  In 
these  and  countless  other  examples,  materials  are  removed  from 
one  place  and  set  down  in  another,  and  in  their  new  position, 
while  acquiring  novel  characters,  they  retain  more  or  less  dis- 
tinctly the  record  of  their  source  and  of  the  conditions  under 
which  their  transference  was  effected. 

In  these  chapters,  reference  has  intentionally  been  avoided  as 
far  as  possible  to  details  that  required  some  knowledge  of  min- 
erals and  rocks,  in  order  that  the  broad  principles  of  geology, 
for  which  such  knowledge  is  not  absolutely  essential,  might  be 
clearly  enforced.  It  is  obvious,  however,  that  as  minerals  and 
rocks  form  the  records  in  which  the  history  of  the  earth  has  been 
preserved,  this  history  cannot  be  followed  into  detail  until  some 
acquaintance  with  these  materials  has  been  made.  What  now 
lies  before  the  reader,  therefore,  in  order  that  he  may  be  able  to 
apply  the  knowledge  he  has  gained  of  geological  processes  to  the 
elucidation  of  former  geological  periods,  is  to  make  himself 
familiar  with  at  least  the  more  common  and  important  minerals 
and  rocks.  This  he  can  only  do  satisfactorily  by  handling  the 


120  GEOLOGY 

objects  themselves,  until  he  acquires  such  an  acquaintance  with 
them  as  to  be  able  to*  recognise  them  where  he  meets  with  them 
in  nature. 

At  first  the  number  and  variety  of  these  objects  may  appear 
to  be  almost  endless,  and  the  learner  may  be  apt  to  despair  of 
ever  mastering  more  than  an  insignificant  portion  of  the  wide 
circle  of  inquiry  and  observation  which  they  present.  But 
though  the  detailed  study  of  this  subject  is  more  than  enough  to 
tax  the  whole  powers  of  the  most  indefatigable  student,  it  is  not 
by  any  means  an  arduous  labour,  and  assuredly  a  most  interest- 
ing one,  to  acquire  so  much  knowledge  of  the  subject  as  to  be 
able  to  follow  intelligently  the  progress  of  geological  investiga- 
tion, and  even  to  take  personal  part  in  it.  This  accordingly  is 
the  task  to  which  he  is  invited  in  the  present  and  following 
chapters. 

SIMPLE  ELEMENTS  COMPOSING  THE  EARTH'S  CRUST. 

Before  considering  the  characters  presented  by  the  various 
rocks  that  form  the  visible  part  of  the  earth's  crust,  we  may  find 
it  of  advantage  to  inquire  into  the  general  chemical  composition 
of  rocks,  for  by  so  doing  we  learn  that  though  the  chemist  has 
detected  more  than  sixty  substances  which  he  has  been  unable  to 
decompose,  and  which  therefore  he  calls  elements,  only  a  small 
proportion  of  these  enter  largely  into  the  composition  of  the 
outer  part  of  the  globe.  In  fact,  there  are  only  about  sixteen 
elements  that  play  an  important  part  as  constituents  of  rocks; 
these  together  constitute  about  ninety-nine  parts  of  the  terres- 
trial crust.  Half  of  them  are  metals;  and  the  other  half  are 
metalloids  or  non-metals,  as  in  the  two  subjoined  lists,  the  most 
abundant  being  in  each  case  placed  first. 

METALLOIDS  OB  NON-METALS.  METALS. 


Oxvcen    , 

Symbol. 
O 

Atomic 
Weight. 
1596 

Aluminium 

Symbol. 
Al 

Atomic 
Weight. 
27  30 

Silicon   

....     Si 

2800 

Calcium    .  . 

Ca 

3990 

Carbon  

C 

11  97 

Magnesium      . 

Me 

2394 

Sulphur   

,  ..  .  .    s 

31  98 

Potassium 

K 

3904 

Hydrogen  .  . 

H 

1  00 

Sodium 

Na 

2299 

Chlorine 

Cl 

35.37 

Iron    

Fe 

5590 

Phosphorus 

p 

30  96 

Mn 

54  SO 

Fluorine    . 

F 

19.10 

Barium    . 

Ba 

136.80 

ELEMENTS  AND  MINERALS  121 

Some  of  those  elements  occur  in  the  free  state,  that  is,  not 
combined  with  any  other  element.  Carbon,  for  instance,  is 
found  pure  in  the  form  of  the  diamond,  and  also  as  graphite. 
But  in  the  great  majority  of  cases,  they  assume  various  combina- 
tions. Most  abundant  are  oxides,  or  compounds  of  oxygen  with 
another  element.  Compounds  of  sulphur  and  a  metal  are 
known  as  sulphides;  and  similar  compounds  with  chlorine  are 
chlorides.  Some  of  the  compounds  form  further  combinations 
with  one  or  more  elements.  Thus  the  acid-forming  oxides  unite 
with  water  to  form  what  are  called  adds,  which,  combining  with 
metallic  oxides  or  bases,  form  with  them  compounds  termed  salts. 
Sulphur  and  oxygen,  for  example,  uniting  in  certain  proportions 
with  water,  constitute  sulphuric  acid  (EbSCh)  which,  parting 
with  its  displaceable  hydrogen  and  combining  with  the  metal 
calcium,  forms  the  salt  known  as  calcium-sulphate,  or  sulphate 
of  lime  (CaSo4). 

METALLOIDS. —  Of  the  non-metallic  elements,  by  far  the  most 
abundant  and  important  is  OXYGEN.  In  its  free  state,  it  exists 
as  a  gas  which  has  been  occasionally  detected  at  active  volcanic 
vents.  But  with  this  rare  exception,  it  is  always  found  mixed 
or  combined  with  one  or  more  elements.  Thus,  mixed  with 
nitrogen,  it  constitutes  the  atmosphere,  of  which  it  forms  not 
less  than  23  per  cent  by  weight.  It  takes  a  still  larger  share  in 
the  composition  of  water,  which  consists  of  88.88  per  cent  of 
oxygen  and  11.12  of  hydrogen.  There  is  a  continual  removal  of 
oxygen  from  air  and  water  in  the  processes  of  weathering  de- 
scribed in  Chapter  II.  Substances  which  can  take  more  of  this 
element  abstract  it  especially  from  damp  air  or  from  water.  A 
knife  or  any  other  piece  of  iron,  for  example,  will  remain  un- 
changed for  an  indefinite  length  of  time  if  kept  in  dry  air ;  but 
as  soon  as  it  is  exposed  to  moisture,  in  which  there  is  always 
some  dissolved  air,  it  begins  to  rust.  The  familiar  brown  rust 
which  slowly  eats  into  the  very  centre  of  the  iron,  is  due  to  a 
chemical  union  of  oxygen  with  the  iron,  forming  what  is  called 
an  Oxide  of  iron  with  water  (see  Oxides).  Among  the  rocks  of 
the  earth's  crust,  a  large  proportion  are  liable  to  undergo  a 
similar  change,  and  so  enormous  has  been  the  extent  of  this 
change  in  the  past  history  of  our  globe,  that  somewhere  about 
one-half  of  the  outer  and  accessible  part  of  the  crust  consists  of 
oxygen,  which  was  probably  at  first  in  the  atmosphere. 


122  GEOLOGY 

Next  in  importance  to  oxygen  among  the  metalloids  is  SILICON, 
which  is  never  met  with  in  the  free  state.  It  has  been  artificial- 
ly obtained,  however,  in  the  form  of  a  dull  brown  powder.  In 
nature,  it  always  occurs  united  with  oxygen,  forming  the  familiar 
substance  known  as  Silica  or  Silicic  Acid  (SiO),  which  consti- 
tutes more  than  a  half  of  all  the  known  part  of  the  earth's  crust. 
Silica  is  indeed  the  fundamental  compound  of  the  crust,  forming 
by  itself  entire  masses  of  rock,  and  entering  as  a  principal  con- 
stituent into  the  majority  of  rocks.  It  occurs  abundantly  as 
the  mineral  Quartz,  the  colourless  transparent  forms  of  which  are 
known  as  rock-crystal  (Fig.  44),  and  also  in  combination  with 
various  metallic  bases  as  the  important  family  of  Silicates  (which 
see).  It  is  present  in  solution  in  most  natural  waters,  both 


Fig.  44. —  Group  of  Quartz-crystals    (Rock-crystals). 

those  of  the  land  and  of  the  sea,  whence  it  is  secreted  by  plants 
(diatoms,  grasses)  and  animals  (radiolarians  and  sponges). 
It  is  thus  carried  by  percolating  water  into  the  heart  of  rocks, 
and  may  be  deposited  in  their  interstices  and  cavities.  Its  hard- 
ness and  durability  eminently  fit  it  for  the  important  part,  it 
plays  in  binding  the  materials  of  rocks  together,  and  enabling 
them  better  to  resist  the  decomposing  effects  of  air  and  water. 

CARBON,  though  found  in  a  nearly  pure  state  in  the  clear  gem 
called  Diamond,  and  also  in  the  black  opaque  mineral  Graphite, 
more  usually  occurs  mixed  with  various  impurities,  as  in  the 
different  kinds  of  coal.  This  element  has  a  high  importance  in 
nature,  because  it  is  the  fundamental  substance  made  use  of  by 
both  plants  ajid  animals  to  build  up  their  structures,  and  be- 


ELEMENTS  AND  MINERALS  123 

cause  it  serves  as  a  bond  of  connection  between  the  organic  and 
the  inorganic  worlds.  In  union  with  oxygen,  Carbon  forms  the 
widely-diffused  gaseous  compound  known  as  Carbon-dioxide 
(CO),  which  occurs  in  the  proportion  of  about  four  parts  in 
every  ten  thousand  parts  of  ordinary  atmospheric  air.  From 
the  air  it  is  abstracted  and  decomposed  by  living  plants  in  pres- 
ence of  sunshine,  the  oxygen  being  in  great  measure  sent  back 
into  the  atmosphere,  while  the  carbon  with  some  oxygen,  nitro- 
gen, and  hydrogen  is  built  up  into  the  various  vegetable  cells 
and  tissues.  When  we  look  at  a  verdant  landscape  or  a  bound- 
less forest,  it  is  a  striking  thought  that  all  this  vegetation  has 
been  chiefly  constructed  out  of  the  small  proportion  of  invisible 
carbon-dioxide  present  in  the  atmosphere.  The  vast  numbers 
of  beds  of  coal  imbedded  in  the  earth's  crust  have,  in  like  man- 
ner, been  derived  from  the  atmosphere  through  the  agency  of 
former  tribes  of  plants.  Not  only  in  beds  of  coal,  but  still  more 
prevalently  in  masses  of  limestone,  carbon  enters  into  the  com- 
position of  rocks.  Carbon-dioxide,  as  was  pointed  out  in  Chapter 
II,  is  abstracted  by  rain  in  passing  from  the  clouds  to  the  earth, 
and  is  also  supplied  by  decomposing  plants  and  animals  in  the 
soil.  It  is  readily  dissolved,  in  water,  and  forms  with  it  car- 
bonic acid,  CO  (OH)  2,  which  has  been  referred  to  as  so  powerful 
a  solvent  of  the  substance  of  many  rocks.  This  acid  unites  with 
a  number  of  alkaline  and  earthy  bases  to  form  the  important 
family  of  Carbonates.  Of  these  the  most  abundant  is  calcium- 
carbonate,  or  carbonate  of  lime  (CaCOs),  which  consists  of  44 
per  cent  carbon-dioxide,  and  56  per  cent  lime.  This  carbonate 
not  only  occurs  abundantly  diffused  through  many  rocks,  but  in 
the  form  of  limestone  builds  up  by  itself  thick  mountainous 
masses  of  rock  many  hundreds  of  square  miles  in  extent.  It  is 
abstracted  by  plants  to  form  calcareous  tufa  (Chapter  V),  but 
far  more  abundantly  by  animals,  especially  by  the  invertebrata, 
as-  exemplified  by  the  familiar  urchins,  corallines,  and  shells  of 
the  sea-shore.  The  limestones  of  the  earth's  crust  appear  to 
have  been  mainly  formed  of  the  calcareous  remains  of  animals. 
Hence  we  perceive  that  the  two  forms  in  which  carbon  has  been 
most  abundantly  stored  up  in  the  earth's  crust  have  been  princi- 
pally due  to  the  action  of  organised  life ;  coal  being  chiefly  carbon 
that  has  been  taken  out  of  the  atmosphere  by  plants,  and  lime- 
stone consisting  of  carbon-dioxide,  to  the  extent  of  nearly  one- 


124  GEOLOGY 

half,  which  has  been  secreted  from  water  by  the  agency  of 
animals. 

SULPHUR  is  found  in  the  free  state,  more  particularly  at  vol- 
canic vents,  in  pale  yellow  crystals  or  in  shapeless  masses  and 
grains ;  but  it  chiefly  occurs  in  combination.  Some  of  its  com- 
pounds are  widely  diffused  among  plants  and  animals.  The 
blackening  of  a  silver  spoon  by  a  boiled  egg  is  an  illustration 
of  this  diffusion,  for  it  arises  from  the  union  of  the  sulphur  in 
the  egg  with  the  metal.  Combinations  with  a  metal  (Sulphides), 
and  combinations  with  a  metal  and  oxygen  (Sulphates)  are  the 
conditions  in  which  sulphur  chiefly  exists. 

HYDROGEN  is  a  gas  which  has  been  detected  in  the  free  state 
at  active  volcanic  vents;  but  otherwise  it  occurs  chiefly  in  com- 
bination with  oxygen  as  the  oxide  water  (EkO),  of  which  it 
constitutes  about  one-ninth,  or  11.12  per  cent  by  weight.  It  also 
enters  into  the  composition  of  plant  and  animal  substances,  and 
forms  with  carbon  the  important  group  of  bodies  known  as  Hy- 
drocarbons, of  which  mineral  oil  and  coal-gas  are  examples.  In 
smaller  quantity,  it  is  found  united  with  sulphur  (sulphuretted 
hydrogen,  H^S),  with  chlorine  (hydrochloric  acid,  HC1),  and  a 
few  other  elements. 

CHLORINE  is  a  transparent  gas  of  a  greenish-yellow  colour,  but 
except  possibly  at  active  volcanic  vents  it  does  not  occur  in  the 
free  state.  United  with  the  alkali  metals  (potassium,  sodium, 
and  magnesium),  it  forms  the  chief  salts  of  sea-water.  The  most 
important  of  these  salts,  Sodium-chloride,  or  common  salt 
(NaCl)  contains  60.64  per  cent  of  chlorine,  and  forms  2.64  per 
cent  by  weight  of  sea-water.  This  salt  is  found  diffused  in  mi- 
croscopic particles  in  the  air,  especially  near  the  sea,  and  beds 
of  it  hundreds  of  feet  thick  occur  in  many  parts  of  the  world 
among  the  sedimentary  rocks  that  constitute  most  of  the  dry 
land. 

PHOSPHORUS  does  not  occur  free;  it  has  so  strong  an  affinity 
for  oxygen  that  it  rapidly  oxidises  on  exposure  to  the  air,  and 
even  melts  and  takes  fire.  Its  most  frequent  combination  is  with 
oxygen  and  calcium,  as  Calcium-phosphate  or  as  phosphate  of 
lime  (Ca3(PO*)2.  Though  for  the  most  part  present  in  mi- 
nute proportions,  it  is  widely  diffused  in  nature.  It  occurs  in 
fresh  and  sea-water,  in  soil  and  in  plants,  especially  in  their 
fruits  and  seeds;  it  is  supplied  by  plants  to  animals  for  the 


ELEMENTS  AND  MINERALS  125 

formation  of  bones,  which  when  burnt  consist  almost  entirely 
of  phosphate  of  lime. 

FLUORINE  also  is  never  met  with  uncombined ;  it  never  unites 
with  oxygen,  forming  in  this  respect  the  sole  exception  among 
the  elements.  Its  most  frequent  combination  as  a  rock  constit- 
uent is  with  calcium,  when  it  forms  the  mineral  Fluor-spar 
(CaFa).  Like  phosphorus,  fluorine  is  widely  diffused  in  minute 
proportions  in  the  waters  of  some  springs,  rivers,  and  the  sea, 
and  in  the  bones  of  animals. 

To  these  metalloids  we  may  add  the  colourless,  tasteless  gas 
NITROGEN,  which,  though  not  largely  present  in  the  earth's  crust, 
constitutes  four-fifths  by  volume  or  77  per  cent  by  weight  of  the 
atmosphere.  It  does  not  enter  into  combination  so  readily  as  the 
other  elements  above  enumerated,  but  it  is  always  found  in  the 
composition  of  plants  and  is  a  constituent  of  many  animal  tissues. 
It  is  the  principal  ingredient  of  the  substance  called  Ammonia, 
which  is  produced  when  moist  organic  matter  is  decomposed  in 
the  air.  In  many  rocks  composed  wholly  or  in  great  part  of 
organic  remains,  such,  for  instance,  as  peat  and  coal,  nitrogen  is 
a  constant  constituent. 

Metals. —  Though  so  large  a  proportion  of  the  known  terres- 
trial elements  are  metals,  these  are  much  less  abundant  in  the 
earth's  crust  than  the  metalloids.  The  most  frequent  are  Alumin- 
ium, Calcium,  and  Magnesium.  The  substances  most  familiar 
to  us  as  metals  occupy  an  altogether  subordinate  part  among 
rocks,  the  most  abundant  of  them  being  Iron. 

ALUMINIUM  never  occurs  in  the  free  state,  but  can  be  artifi- 
cially separated  from  its  compounds,  when  it  is  seen  to  be  a  white 
light,  malleable  metal.  It  is  almost  always  united  with  oxygen 
as  the  oxide  of  Alumina  (AbOa),  which  occurs  crystallised  as  the 
ruby  and  sapphire,  but  is  for  the  most  part  united  with  silica, 
and  in  this  form  constitutes  the  basis  of  the  great  family  of 
minerals  known  as  the  Silicates  of  Alumina,  or  Aluminous  Sili- 
cates. These  silicates  generally  contain  some  other  ingredient 
which  is  more  liable  to  decomposition,  and  when  they  decay  and 
their  more  soluble  parts  are  removed,  they  pass  into  clay,  which 
consists  chiefly  of  hydrated  silicate  of  alumina. 

CALCIUM  is  not  met  with  uncombined,  but  has  been  artificially 
isolated  and  found  to  be  a  light  yellowish  metal,  between  gold 
and  lead  in  hardness.  It  occurs  in  nature  chiefly  combined  with 


126  GEOLOGY 

carbonic  acid  as  a  carbonate,  and  also  with  sulphuric  acid  as 
a  sulphate,  to  both  of  which  substances  reference  has  already 
been  made;  and  it  is  also  present  in  many  silicates.  So 
abundant  is  Calcium-carbonate  or  carbonate  of  lime  in  nature 
that  it  may  be  found  in  most  natural  waters,  which  dissolve  it 
and  carry  it  in  solution  into  the  sea.  Its  presence  in  rocks  may 
be  detected  by  a  drop  of  any  mineral  acid,  when  the  liberated 
carbon-dioxide  escapes  as  a  gas  with  brisk  effervescence.  Calcium- 
sulphate  is  likewise  a  common  constituent  of  terrestrial  waters, 
especially  of  those  which  in  household  management  are  called 
hard;  it  constitutes  not  less  than  3.6  per  cent  of  the  salts  in 
ordinary  sea-water,  and  when  sea-water  is  evaporated  this  sul- 
phate (gypsum),  being  least  soluble,  is  the  first  to  be  precipi- 
tated in  minute  crystals  resembling  in  shape  those  shown  in 
Fig.  62. 

MAGNESIUM  is  likewise  only  isolated  artificially,  when  it  ap- 
pears as  a  soft,  silver-white,  malleable  and  ductile  metal.  It 
occurs  in  sea-water  combined  with  chlorine  as  Magnesium-chlo- 
ride, which  constitutes  10.8  per  cent  of  the  total  proportion  of 
salts.  It  unites  with  carbonic  acid  as  a  carbonate,  which  with 
carbonate  of  lime  forms  the  widely  diffused  rock  called  magne- 
sian  limestone  or  Dolomite;  it  also  enters  into  the  composition 
of  the  Magnesian  Silicates  which  are  only  second  in  importance 
to  those  of  alumina. 

POTASSIUM  and  SODIUM  (alkali  metals)  are  only  obtainable  in 
the  free  state  by  chemical  processes,  when  they  are  found  to  be 
white  brittle  metals  that  float  on  water,  and  rapidly  oxidise  if 
exposed  to  the  air.  Combined  with  chlorine,  Sodium  forms  the 
familiar  chloride  known  as  common  salt,  which  constitutes  77.7 
per  cent  of  the  salts  of  sea-water,  is  abundantly  present  in  salt 
lakes,  and  occurs  also  in  extensive  beds  among  the  rocks  of  the 
dry  land.  Potassium-chloride  likewise  occurs  in  the  sea  and 
may  be  obtained  from  the  ashes  of  burnt  sea-weed.  Enor- 
mous deposits  of  it,  combined  with  chlorides  of  sodium  and 
magnesium,  have  been  met  with  in  Germany  (Stassfurt).  Pot- 
assium also  exists  in  the  sea  in  combination  with  sulphuric  acid 
as  potassium-sulphate  or  sulphate  of  potash,  which  amounts  to 
about  2.4  of  the  total  salts  of  sea-water.  Sulphates  of  potassium, 
sodium,  magnesium,  and  calcium  form  thick  masses  of  rock  in 
the  Stassfurt  deposits.  Potassium  and  sodium  in  combination 


ELEMENTS  AND  MINERALS  127 

with  silica  form  silicates  which  enter  largely  into  the  composi- 
tion of  many  rocks.  They  are  readily  attacked  by  water  contain- 
ing carbonic  acid,  giving  rise  to  what  are  called  carbonates  of  the 
alkalies,  or  alkaline  carbonates,  which  are  removed  in  solution. 
By  this  means,  carbonate  of  potash  is  introduced  into  soil,  where 
it  is  taken  up  by  plants  into  their  leaves  and  succulent  parts. 
When  wood  is  burnt,  this  carbonate  in  considerable  quantity  may 
be  dissolved  with  water  out  of  the  ash. 

IRON"  is  found  in  the  free  or  native  state  in  minute  grains 
(rarely  in  large  blocks)  in  some  volcanic  rocks,  also  in  granules 
of  "  cosmic  dust,"  probably  of  meteoric  origin,  and  in  fragments 
of  various  size  which  have  undoubtedly  fallen  upon  the  earth's 
surface  from  the  regions  of  space.  There  is  reason  to  believe 
that  much  of  the  solid  interior  of  the  globe  may  consist  of  na- 
tive iron  and  other  metals.  But  it  is  in  combination  that  iron 
is  chiefly  of  importance  in  the  earth's  crust.  It  has  united  with 
oxygen  to  form  several  abundant  oxides.  The  protoxide  or 
ferrous  oxide  (FeO)  contains  the  lowest  proportion  of  oxygen, 
and  being,  therefore,  prone  to  take  up  more,  gives  rise 
to  many  of  the  processes  of  decay  included  under  the  general 
name  of  Weathering.  It  is  readily  dissolved  by  organic 
and  other  acids,  and  is  then  removed  in  solution,  but  on  exposure 
rapidly  oxidises  and  passes  into  the  highest  oxide,  known  as  the 
peroxide  or  sesquioxide  of  iron  or  ferric  oxide  (FeaOs),  which, 
being  the  permanent  insoluble  form,  is  found  abundantly  among 
the  rocks  of  the  earth's  crust.  Iron  is  the  great  colouring  matter 
of  nature;  its  protoxide  compounds  give  greenish  hues  to  many 
rocks,  while  its  peroxide  colours  them  various  shades  of  red 
which,  when  the  peroxide  is  combined  with  water,  pass  into  many 
tints  of  brown,  orange,  and  yellow. 

MANGANESE  is  commonly  associated  with  iron  in  minute  pro- 
portion in  many  lavas  and  other  crystalline  rocks;  its  oxides 
resemble  those  of  iron  in  their  modes  of  occurrence. 

BARIUM  and  Calcium  are  called  metals  of  the  alkaline  earths. 
The  former  can  only  be  obtained  in  a  free  state  by  artificial 
means,  when  it  appears  as  a  pale  yellow  very  heavy  metal  which 
rapidly  tarnishes.  In  nature  it  chiefly  occurs  as  the  sulphate, 
Barytes,  or  heavy  spar  (BaSCh),  a  mineral  of  frequent  occur- 
rence in  veins  associated  with  metallic  ores. 


128  GEOLOGY 

MINERALS  OF  CHIEF  IMPORTANCE  IN  THE  EARTH'S  CRUST. 

Passing  now  from  the  simple  elements,  we  have  next  to  note 
the  mineral  forms  in  which  they  appear  as  constituents  of 
the  earth's  crust.  A  mineral  may  be  defined  as  an  inorganic 
substance,  having  theoretically  a  definite  chemical  composition,, 
and  in  most  cases  also  a  certain  geometrical  form.  It  may 
consist  of  only  one  element,  for  example,  the  diamond,  sulphur, 
and  the  native  metals,  gold,  silver,  copper,  etc.  But  in  the 
vast  majority  of  cases,  minerals  consist  of  at  least  two,  usually 
more,  elements  in  definite  chemical  proportions.  In  the  follow- 
ing short  list  of  the  more  important  minerals  of  the  earth's 
crust  they  are  arranged  chemically,  according  to  the  predom- 
inant element  in  them,  or  the  manner  in  which  the  combinations 
of  the  elements  have  taken  place,  so  that  their  leading  features 
of  composition  may  be  at  once  perceived.  The  two  elements, 
Carbon  and  Sulphur,  in  their  native  or  uncombined  state, 
sometimes  form  considerable  masses  of  rock.  Some  of  the  native 
metals  also  may  be  enumerated  as  rock-constituents  when  they 
occur  in  sufficient  quantities  to  be  commercially  important. 
Gold,  for  example,  is  found  in  grains  and  strings,  in  veins  of 

quartz,  and  in  irregular  pieces 
or  nuggets  dispersed  through  the 
gravel  deposits  of  regions  where 
gold-bearing  quartz-veins  traverse 
the  solid  rocks.  Omitting,  how- 
ever, the  minerals  formed  of  a 
single  element,  we  may  pass  on  to 
combinations  of  two  or  more  ele- 
ments, and  consider  first  those  in 
which  oxygen  is  combined  with 
some  other  element,  forming  what 

Fig.  45.- Ca1cite< Iceland  spar),  are  commonly  grouped  together 
showing  its  characteristic  rhom-  as  Oxides.  Then  Will  come  the 

bohedral   cleavage.  Silicates,  or  combinations  of  silica 

with  one  or  more  bases,  followed  by  the  Carbonates,  or  com- 
binations of  carbon-dioxide  with  some  base;  the  Sulphates,  or 
compounds  of  sulphuric  acid  and  a  base;  the  Fluorides,  or 
compounds  of  fluorine  and  a  metal ;  the  Chlorides,  or  compounds 


ELEMENTS  AND  MINERALS  129 

of  chlorine  and  a  metal;  and  the  Sulphides,  or  compounds  of 
sulphur  and  a  metal. 

It  cannot  be  too  strongly  impressed  upon  the  mind  of  the 
learner  that  no  mere  description  in  books  will  suffice  to  make 
him  familiar  with  minerals  and  rocks.  He  ought  to  handle 
actual  specimens  of  these  objects  and  identify  for  himself  the 
several  characters  which  he  finds  assigned  to  them  in  books. 

One  of  the  most  obvious  features  in  a  crystal  of  any  mineral 
is  the  regular  and  sharply-defined  edges  and  corners  which  it 
presents.  Take  a  piece  of  rock-crystal  or  quartz,  for  example 
(Fig.  44),  and  you  will  find  it  to  consist  of  six  sides  or  faces, 
forming  what  is  called  a  prism,  and  bevelled  off  at  the  end  into 
a  six-sided  cone,  called  a  pyramid.  If  you  examine  a  large 
collection  of  similar  crystals  you  may  find  no  two  of  them 
exactly  alike,  yet  they  agree  in  presenting  a  six-sided  figure. 
Again,  procure  a  piece  of  the  common  mineral  calcite,  either 
a  whole  crystal  (Figs.  59,  60),  or  a  portion  of  a  crystalline 
mass  (Fig.  45) ;  break  it  and  you  will  find  each  fragment  to 
possess  the  same  form,  that  x  of  a  rhombohedron ;  crush  one 
of  these  fragments  and  you  will  observe  that  each  little  grain 
of  the  powder  preserv.es  the  same  shape.  The  rhombohedron, 
therefore,  is  called  the  fundamental  crystalline  form  of  the 
mineral.  The  property  so  strikingly  shown  in  calcite,  of  break- 
ing along  definite  crystalline  planes,  is  termed  cleavage.  So 
perfect  is  the  cleavage  of  calcite,  that  the  crystallised  mineral  can 
hardly  be  broken,  except  along  the  planes  that  define  the  rhom- 
bohedron. Many  minerals  cleave  more  or  less  easily  in  one  or 
more  directions,  and  break  irregularly  in  others.  The  cleavage 
affords  a  guide  to  the  proper  crystalline  form  of  a  mineral. 

Though  there  are  many  hundreds  of  varieties  of  crystalline 
form,  they  may  all  be  reduced  to  six  primary  types  or  systems. 
These  are  distinguished  from  each  other  by  the  number  and 
position  of  their  axes,  which  are  mathematical  straight  lines, 
intersecting  each  other  in  the  interior  of  a  crystal,  and  connect- 
ing the  centre  of  opposite  flat  faces  of  the  crystal,  or  opposite 
angles  or  corners.  The  six  systems,  with  their  axes,  are  enumer- 
ated in  the  subjoined  list. 

I.  Isometric  (monometric,  cubical,  tesseral,  regular).  In  this  sys- 
tem there  are  three  axes  which  are  of  the  same  length,  and 
intersect  each  other  at  a  right  angle.  The  cube,  octahedron, 


130 


GEOLOGY 


and  dodecahedron  are  examples  (Fig.  46).  Crystals  of  this 
system  are  distinguished  by  their  symmetry,  their  length, 
breadth,  and  thickness  being  equal.  Common  salt,  fluor-spar 
(Fig.  04),  and  magnetite  (Fig.  54)  are  illustrations. 


Fig.  46. —  Cube  (a),  octahedron  (&),  dodecahedron  (c). 

II.  Tetragonal  (dimetric).  The  axes  are  three  in  number,  and  inter- 
sect each  other  at  a  right  angle,  but  one  of  them,  called  the 
vertical  axis,  is  longer  or  shorter  than  the  other  two,  which 
are  lateral  axes.  Hence  a  crystal  belonging  to  this  system  may 
either  be  oblong  or  squat  (Fig.  47). 


Fig.     47. —  Tetragonal     prism     (6) 
and   pyramid    (a). 


Fig.    48. — Orthorhombic 
prism. 


III.  Orthorhombic  (trimetric)  has  the  three  axes  intersecting  each 
other  at  a  right  angle,  but  all  of  unequal  lengths.  The  rec- 
tangular and  rhombic  prisms  and  the  rhombic  octahedron  be- 
long to  this  system  (Fig.  48). 


Fig.   49. —  Hexagonal   prism    (a),   rhombohedron    (6),   and 
scalenohedron   (c). 

IV.  Hexagonal.     This  is  the  only  system  with   four  axes    (Fig.  49). 
The  lateral  axes  are  all  equal,  intersect  at  right  angles  the  ver- 


ELEMENTS  AND  MINERALS  131 

tical  axis  (which  is  longer  or  shorter  than  they  are),  and  form 
with  each  other  angles  of  60°.  Water,  for  instance,  crystallises 
in  this  system,  and  the  six-rayed  star  of  a  snow-flake  is  an 
illustration  of  the  way  in  which  the  lateral  axes  are  placed. 
Quartz  is  an  example  (Fig.  44),  also  calcite  (Figs.  59,  60). 


Fig.  50. —  Monoclinic  prism.  Fig.  51. — Triclinic  prism. 

Crystal  of  Augite.  Crystal  of  Albite  felspar. 

V.  Monoclinic,  with  all  the  axes  of  unequal  length.  One  of  the 
lateral  axes  cuts  the  vertical  axis  at  a  right  angle,  the  other 
intersects  the  vertical  axis  obliquely.  Augite  (Fig.  50),  Horn- 
blende (Fig.  57),  and  Gypsum  (Fig.  62)  are  examples. 
VI.  Triclinic,  the  most  unsymmetrical  of  all  the  systems,  all  the  axes 
being  unequal  and  placed  obliquely  to  each  other  (Fig.  51). 

Every  mineral  that  takes  a  crystalline  form  belongs  to  one  or 
other  of  these  six  systems,  and  through  all  its  varieties  of  ex- 
ternal form  the  fundamental  relations  of  the  a'xes  remain  •un- 
changed. 

Some  minerals  have  crystallised  out  of  solutions  in  water. 
How  this  may  take  place  can  be  profitably  studied  by  dissolving 
salt,  sugar,  or  alum  in  water,  and  watching  how  the  crystals  of 
these  substances  gradually  shape  themselves  out  of  the  con- 
centrated solution,  each  according  to  its  own  crystalline  pat- 
tern. Other  minerals  have  crystallised  from  hot  vapours  (sub- 
limation), as  may  be  observed  at  the  fissures  of  an  active  vol- 
cano. Other  minerals  have  crystallised  out  of  molten  solu- 
tions, as  in  the  case  of  lava.  Thoroughly  fused  lava  is  a  glassy 
or  vitreous  solution  of  all  the  mineral  substances  that  enter  into 
the  composition  of  the  rock,  and  when  the  lava  cools,  the 
various  minerals  crystallise  out  of  it,  those  that  are  least  fusible 
taking  form  first,  the  most  fusible  appearing  last ;  but  a  residue 
of  non-crystalline  glass  sometimes  remaining  even  when  the  rock 
has  solidified. 

It  is  evident  that  minerals  can  only  form  perfect  crystals 


132  GEOLOGY 

where  they  have  room  and  time  to  crystallise.  But  where  they 
are  crowded  together,  and  where  the  solution  in  which  they  are 
dissolved  dries  or  cools  too  rapidly,  their  regular  and  sym- 
metrical growth  is  arrested.  They  then  form  only  imperfect 
crystals,  but  their  internal  structure  ft  crystalline,  and  if  ex- 
amined carefully  will  be  found  to  show  that  in  the  attempt  to 
form  definite  crystals  each  mineral  has  followed  its  own  crystal- 
line type.  These  characters  are  of  much  importance  in  the 
study  of  rocks,  for  rocks  are  only  large  aggregates  of  minerals, 
wherein  definite  crystals  are  exceptional,  though  the  structure 
of  the  whole  mass  may  still  be  quite  crystalline. 

But  minerals  also  occur  in  various  indefinite  or  non-crystal- 
line shapes.  Sometimes  they  are  -fibrous  or  disposed  in  minute 
fibre-like  threads  (Fig.  56)  ;  or  concretionary  when  they  have 
been  aggregated  into  various  irregular  concretions  of  globular, 
kidney-shaped,  grape-like,  or  other  imitative  shapes  (Figs.  61, 
64,  65,  75)  ;  or  stalactitic  (Fig.  20)  when  they  have  been  de- 
posited in  pendent  forms  like  stalactites;  or  amorphous  when 
they  have  no  definite  shape  of  any  kind,  as,  for  instance,  in 
massive  ironstone. 

Oxides  occur  abundantly  as  minerals.  The  most  important 
are  those  of  Silicon  (Quartz)  and  Iron  (Hematite,  Limonite, 
Magnetite,  Titanic  Iron. 

QUARTZ  (Silica,  Silicic  Acid,  SiO),  already  alluded  to,  is  the 
most  abundant  mineral  in  the  earth's  crust.  It  occurs  crystal- 
lised, also  in  various  crystalline  and  non-crystalline  varieties. 
In  the  crystallised  form  as  common  quartz  it  is,  when  pure,  clear 
and  glassy,  but  is  often  coloured  yellow,  red,  green,  brown,  or 
black,  from  various  impurities.  It  crystallises  in  the  six-sided 
prisms  and  pyramids  above  referred  to,  the  clear  colourless  vari- 
eties being  rock-crystal  (Fig.  44).  When  purple  it  is  called 
amethyst;  yellow  and  smoke-colour  varieties,  found  among  the 
Grampian  Mountains,  are  popularly  known  as  Cairngorm  stones. 
In  many  places,  silica  has  been  deposited  as  chalcedony,  in 
translucent  masses  with  a  waxy  lustre,  and  pale  grey,  blue, 
brown,  red,  or  black  colours.  Deposits  of  this  kind  are  not  in- 
frequent among  the  cavities  of  rocks.  The  common  pebbles  and 
agates  with  concentric  bands  of  different  colours  are  examples 
of  chalcedony,  and  show  how  the  successive  layers  have  been  de- 
posited from  the  walls  of  the  cavity  inwards  to  the  centre  which 


ELEMENTS  AND  MINERALS  133 

is  often  filled  with  crystalline  quartz    (Fig.   52).     The  dark 
opaque  varieties  are  called  jasper. 

Quartz  can  be  usually  recognised  by  its  vitreous  lustre  and 
hardness;  it  cannot  be  scratched  with  a  knife,  but  easily 
scratches  glass,  and  it  is  not  soluble  in  the  ordinary  acids. 
It  is  an  essential  constituent  of  many  rocks,  such  as  granite  and 
sandstone.  Silica  being  dissolved  by  natural  waters,  especially 
where  organic  acids  or  alkaline  carbonates  are  present,  is  intro- 
duced by  permeating  water  into  the  heart  of  even  the  most  solid 
rocks.  Hence  it  is  found  abundantly  in  strings  and  veins 
traversing  rocks,  also  in  cavities  and  replacing  the  forms  of 
plants  and  animals  imbedded  in  sedimentary  deposits.  Soluble 
silica  is  abstracted  by  some  plants  and  animals  and  built  up  into 
their  organic  structures  (diatoms,  radiolarians,  sponges). 


Fig.  52. —  Section  of  a  pebble  of  chalcedony.  The  outer  banded  layers 
are  chalcedony,  the  interior  being  nearly  filled  up  with  crystalline 
quartz. 

Four  minerals  composed  of  oxides  of  Iron  occur  abundantly 
among  rocks.  The  peroxide  is  found  in  two  frequent  forms,  one 
without  water  (Haematite),  the  other  with  water  (Limonite). 
The  peroxide  and  protoxide  combine  to  form  Magnetite,  and  a 
mixture  of  the  peroxide  with  the  peroxide  of  the  metal  titanium 
gives  Titanic  Iron. 

HEMATITE  or  Specular  Iron  (Fe^O^  Fe70030)  occurs  in 
rhombohedral  crystals  that  can  with  difficulty  be  scratched  with 
a  knife ;  but  is  more  usually  found  in  a  massive  condition  with  a 
compact,  fibrous,  or  granular  texture,  and  dark  steel-gray  or  iron- 
black  colour,  which  becomes  bright  red  when  the  mineral  is 
scratched  or  powdered.  The  earthy  kinds  are  red  in  colour,  and 


134 


GEOLOGY 


it  is  in  this  earthy  form  that  haematite  plays  so  important  a  part 
as  a  colouring  material  in  nature.  Red  sandstone,  for  example, 
owes  its  red  colour  to  a  deposit  of  earthy  peroxide  of  iron  round 

the  grains  of  sand.  Hema- 
tite occurs  crystallised  in  fis- 
sures of  lavas  as  a  product  of 
the  hot  vapours  that  escape  at 
these  places;  but  is  more 
abundant  in  beds  and  concre- 
tionary masses  (Fig.  53) 
among  various  rocks. 

LIMONITE   or   Brown   Iron- 
re  differs  from  Hematite  in 

and     the     internal     crystalline  being    rather    softer,    in    con- 
taining more  than  14  per  cent 

of  water  which  is  combined  with  the  iron  to  form  the  hydrated 
peroxide,  in  being  usually  massive  or  earthy,  in  presenting  a  dark 
brown  to  yellow  colour  (ochre),  and  in  giving  a  yellowish-brown 
to  dull  yellow  powder  when  scratched  or  bruised.  It  may  be  seen 
in  the  course  of  being  deposited  at  the  present  time  through  the 
action  of  vegetation  in  bogs  and  also  in  lakes,  hence  its  name 
of  Bog-iron  ore,  likewise  in  springs  and  streams  where  the  water 
carries  much  sulphate  of  iron.  The  common  yellow  and  brown 
colours  of  sandstones  and  many  other  rocks  are  generally  due  to 
the  presence  of  this  mineral. 


Fig.  54. —  Octahedral  crystals  of  magnetite   in  chlorite  schist. 

MAGNETITE  (FeoO*)  occurs  crystallised  in  octahedrons  and 
dodecahedrons  of  an  iron-black  colour,  giving  a  black  powder 
when  scratched.  It  is  found  abundantly  in  many  rocks  (schists, 
lavas,  etc.),  sometimes  in  large  crystals  (Fig.  54),  sometimes  in 
such  minute  form  as  can  only  be  detected  with  the  microscope. 


ELEMENTS  AND  MINERALS  135 

It  also  forms  extensive  beds  of  a  massive  structure.  Its  presence 
in  rocks  may  be  detected  by  its  influence  on  a  magnetised  needle. 
By  pounding  basalt  and  some  other  rocks  down  to  powder, 
minute  crystals  and  grains  of  magnetite  may  be  extracted  with 
a  magnet. 

TITANIC  IRON  (FeTi^Oa)  occurs  in  iron-black  crystals  like 
those  of  haematite  from  which  they  may  be  distinguished  by  the 
dark  colour  and  metallic  lustre  of  its  surface  when  scratched. 
Though  it  occurs  in  beds  and  veins  in  certain  kinds  of  rock 
(schists,  serpentine,  syenite),  its  most  generally  diffused  condi- 
tion is  in  minute  crystals  and  grains  scattered  through  many 
crystalline  rocks  (basalt,  diabase,  etc.). 

MANGANESE  OXIDES  are  commonly  associated  with  those  of 
iron  in  rocks.  They  are  liable  to  be  deposited  in  the  form  of 
bog-manganese,  under  condi- 
tions similar  to  those  in 
which  bog-iron  is  thrown  ^ 
down.  Earthy  manganese  Jj 
oxide  (wad)  not  infrequent- 
ly appears  between  the  joints 
of  fine-grained  rocks  in  arbor- 
escent forms  that  look  so  like 
plants  as  to  have  been  often 
mistaken  for  vegetable  re- 
mains. These  plant-like  de- 
posits are  called  Dendrites  or 
dendritic  markings  (Fig.  55). 

SILICATES.  —  Compounds 
of  Silica  with  various  bases 
form  by  far  the  most  numer- 
ous and  abundant  series  of 
minerals  in  the  earth's  crust. 
They  may  be  grouped  accord- 
ing to  the  chief  metallic  base  Fig  55._  Dendritic  markings  due  to 
in  their  composition,  the  most  arborescent  deposit  of  earthy  man- 
important  are  the  Silicates 

of  Alumina,  and  the  Silicates  of  Magnesia.  Of  the 
aluminous  silicates  we  need  consider  here  only  the  Felspars, 
Zeolites,  and  Mica.  Among  the  magnesian  silicates  it  will  be 
enough  to  note  the  leading  characters  of  Hornblende,  Augite, 


136  GEOLOGY 

Olivine,  Talc,  Chlorite,  and  Serpentine.  When  the  learner  has 
made  himself  so  familiar  with  these  as  to  be  able  readily  to 
recognise  them,  he  may  proceed  to  the  examination  of  others,  of 
which  he  will  find  descriptions  in  treatises  on  Mineralogy  and  in 
more  advanced  text-books  of  Geology. 

FELSPARS. —  This  family  of  minerals  plays  an  important  part 
in  the  construction  of  the  earth's  crust,  for  it  constitutes  the 
largest  part  of  the  crystalline  rocks,  which,  like  lava,  have  been 
erupted  from  below;  is  found  abundantly  in  the  great  series  of 
schists ;  and  by  decomposition  has  given  rise  to  the  clays,  out  of 
which  so  many  sedimentary  rocks  have  been  formed.  The 
felspars  are  divided  into  two  series,  according  to  crystalline  form. 

Orthoclase  or  potash-felspar  contains  about  16.89  per  cent 
of  potash,  crystallises  in  monoclinic  or  oblique  rhombic  prisms, 
but  also  occurs  massive;  is.  white,  gray,  or  pink  in  colour;  has 
a  glassy  lustre ;  can  with  difficulty  be  scratched  with  a  knife,  but 
easily  with  quartz.  Associated  with  quartz,  it  is  an  abundant 
ingredient  of  many  ancient  crystalline  rocks  (granite,  felsite, 
gneiss,  etc.)  In  the  form  of  sanidine  it  is  an  essential  constitu- 
ent of  many  modern  volcanic  rocks. 

Plagioclase. —  Under  this  name  are  grouped  several  species 
of  felspar  which,  differing  much  from  each  other  in  chemical 
composition,  agree  in  crystallising  in  the  same  type  or  system, 
which  is  that  of  a  triclinic  or  oblique  rhomboidal  prism.  As 
abundant  ingredients  of  rocks  they  commonly  appear  as  clear, 
colourless,  or  white  glassy  strips,  on  the  flat  faces  of  which 
a  fine  minute  parallel  ruling  may  be  detected  with  the  naked  eye, 
or  with  a  lens.  This  striation  or  lamellation  is  a  distinctive 
character,  which  proves  the  crystals  in  which  it  occurs  not  to  be 
orthoclase.  The  plagioclase  felspars  occur  as  essential  constitu- 
ents of  many  volcanic  rocks,  and  also  among  ancient  eruptive 
masses  and  schists.  Among  them  are  Microcline  (a  potash- 
felspar),  with  15  per  cent  of  potash;  Albite  or  Soda-felspar,  con- 
taining nearly  12  per  cent  of  soda  (Fig.  51)  ;  Anorthite  or  Lime- 
felspar,  with  20.10  per  cent  of  lime;  Soda-lime  felspar, 
Lime-soda  felspar  —  a  group  of  felspars  containing  variable 
proportions  of  soda,  lime,  and  sometimes  potash ;  the  chief  varie- 
ties are  Oligoclase  (Silica,  62-65  per  cent),  Andesine  (Silica, 
58-61  per  cent),  Labradorite  (Silica,  50-56  per  cent). 

ZEOLITES,  a  characteristic  family  of  minerals,  composed  essen- 


ELEMENTS  AND  MINERALS  137 

tially  of  silicate  of  alumina  and  some  alkali  with  water;  often 
marked  by  a  peculiar  pearly  lustre,  especially  on  certain  planes 
of  cleavage ;  usually  found  fillin'g  up  cavities  in  rocks  where  they 
have  been  deposited  from  solution  in  water.  Some  of  the  species 
commonly  crystallise  in  fine  needles  or  silky  tufts.  The  zeolites 
have  obviously  been  formed  from  the  decomposition  of  other 
minerals,  particularly  felspars.  They  are  especially  abundant  in 
the  steam-cells  of  old  lavas  in  which  plagioclase  felspars  prevail, 
either  lining  the  walls  of  the  cavities,  and  shooting  out  in  crystals 
or  fibres  towards  the  centre  (Fig.  56),  or  filling  the  cavities  up 
entirely. 

MICA,   a  group   of  minerals    (monoclinic)    specially   distin- 
guished by  their  ready  cleavage  into  thin,  parallel,  usually  elastic 


Fig.  56. —  Cavity  in  a  lava,  filled  with  zeolite  which  has  crystallised  in 
long  slender  needles. 

silvery  Iamina3.  They  are  aluminous  silicates  with  potash 
(soda),  or  with  magnesia  and  ferrous  oxide,  and  always  with 
water.  They  occur  as  essential  constituents  of  granite,  gneiss, 
and  many  other  eruptive  and  schistose  rocks,  also  in  worn 
spangles  in  many  sedimentary  strata-  (micaceous  sandstone). 
Among  their  varieties  the  two  most  important  are  Muscovite 
(white  mica,  potash-mica),  and  Black  mica  (magnesia-mica,. 
Biotite) . 

HORNBLENDE  or  AMPHIBOLE,  a  silicate  of  magnesia,  with  lime, 
iron-oxides,  and  sometimes  alumina,  occurs  in  monoclinic 
(oblique  rhombic)  prisms,  also  columnar,  fibrous,  and  massive. 
It  is  divisible  into  (1)  a  group  of  pale-coloured  varieties,  con- 
taining little  or  no  alumina,  white  or  pale  green  in  colour,  often 
fibrous  (Tremolite.,  Actinolite,  Asbestus),  found  more  particu- 


138 


GEOLOGY 


Fig.  57. — Horn- 
blende crystal. 


larly  among  gneisses,  marbles,  and  associated  rocks,  and  (2)  a 
dark  group  containing  5  to  18  per  cent  of 
alumina,  which  replaces  the  other  bases ;  dark 
green  to  black  in  colour,  in  stout,  dumpy 
prisms  (Fig.  57),  and  in  columnar  or  bladed 
aggregates  (Common  hornblende).  Abun- 
dant in  many  eruptive  rocks,  and  also  form- 
ing almost  entire  beds  of  rock  among  the 
crystalline  schists. 

AUGITE  (PYROXENE),  in  composition  re- 
sembles hornblende;  indeed,  they  are  only 
different  forms  of  the  same  substance;  differing  slightly 
in  crystalline  form,  hornblende  being  the  result  of  slow  and 
augite  of  rapid  crystallisation.  Many  rocks  in  which  the 
dark  silicate  was  originally  augite  have  that  mineral  now 
replaced  by  hornblende,  as  the  result  of  a  gradual  internal 
alteration.  Like  hornblende,  augite  occurs  in  two  groups:  (1) 
pale  non-aluminous,  found  more  especially  among  gneisses,  mar- 
bles, and  associated  rocks;  and  (2)  dark  green  or  black  (Fig. 
50),  occurring  abundantly  in  many  eruptive  rocks,  such  as  black 
heavy  lavas  (basalts,  etc.). 

OLIVINE  (PERIDOT)  (SKM1.01,  MgO  49.16,  FeO  9.83)  oc- 
curs in  small  orthorhombic  prisms  and  glassy  grains  in  basalts 
and  other  lavas ;  of  a  pale  yellowish-green  or  olive-green  colour, 
whence  its  name.  These  grains  can  often  be 
readily  detected  on  the  black  ground  of  the 
rock,  through  which  they  are  abundantly 
dispersed.  Olivine  is  liable  to  alteration, 
and  especially  to  conversion  into  serpentine 
by  the  influence  of  percolating  water 
(Fig.  58). 

CHLORITE  (SiO*  25-28,  AkO  19-23,  FeO 
15-29,  MgO  13-25,  H*0  9-12)  is  a  dark  olive- 
green  hydrated  magnesian  silicate.  It  is  so 
soft  as  to  be  easily  scratched  with  the  nail, 
and  occurs  in  small  six-sided  tables,  also  in 
various  scaly  and  tufted  aggregations  dif- 
fused through  certain  rocks.  It  appears 
generally  to  be  the  result  of  the  alteration  of 
some  previous  anhydrous  magnesian  silicate,  such  as  hornblende. 


Fig.  58— Olivine 
crystal;  the  light 
portions  repre- 
sent the  unde- 
composed  miner- 
al, the  shaded 
parts  show  the 
conversion  of  the 
olivine  into  ser- 
pentine. 


ELEMENTS  AND  MINERALS  139 

SERPENTINE  (Mg^O-fglLO)  is  another  hydrated  mag- 
nesian  silicate,  containing  a  little  protoxide  of  iron  and  alumina, 
usually  massive,  dark  green  but  often  mottled  with  red.  It 
occurs  in  thick  beds  among  schists,  is  often  associated  with 
limestones,  and  may  be  looked  for  in  all  rocks  that  contain 
olivine,  of  the  alteration  of  which  it  is  often  the  result.  In 
many  serpentines,  traces  of  the  original  olivine  crystals  can  be 
detected. 

CARBONATES. —  Though  these  are  abundant  in  nature,  only 
three  of  them  require  notice  here  as  important  constituents  of 
the  earth's  crust,  those  of  lime,  magnesia  and  lime,  and  iron. 

CALCITE  (calcium-carbonate,  carbonate  of  lime,  CaCO)  crys- 
tallises in  the  hexagonal  system,  and  has  for  its  fundamental 


Fig.  59.—  Calcite  in  the  form  of  "  nail-head  spar." 


crystalline  form  the  rhombohedron,  as  already  mentioned. 
When  it  is  quite  pure  it  is  transparent  (Iceland  spar,  Fig. 
45),  with  the  lustre  of  glass:  but  more  usually  is  translucent 
or  opaque  and  white.  Its  crystals,  where  the  chief  axis  is  shorter 
than  the  others,  sometimes  take  the  form  of  flat  rhombohedrons 
(nail-head  spar,  Fig.  59)  ;  where,  on  the  other  hand,  that  axis 
is  elongated,  they  present  pointed  pyramids  (scalenohedrons, 
dog-tooth  spar,  Fig.  60).  The  mineral  occurs  also  in  fibrous, 
granular,  and  compact  forms.  The  decomposition  of  silicates 
containing  lime  by  permeating  water  gives  rise  to  calcium-car- 
bonate, which  is  removed  in  solution.  Being  readily  soluble  in 
water  containing  carbonic  acid,  this  carbonate  is  found  in  almost 
all  natural  waters,  by  which  it  is  introduced  into  the  cavities  of 
rocks.  Some  plants  and  many  animals  secrete  large  quantities  of 
carbonate  of  lime,  and  their  remains  are  aggregated  into  beds  of 
limestone,  which  is  a  massive  and  more  or  less  impure  form  of 


140  GEOLOGY 

calcite.  Calcite  is  easily  scratched  with  a  knife,  and  is  character- 
ised by  its  abundant  effervescence  when  acid  is  dropped  upon  it. 

A  less  frequent  and  stable  form  of  calcium-carbonate  is 
ARAGONITE  which  crystallises  in  orthorhombic  forms,  but  is  more 
usually  found  in  globular,  dendritic,  coral-like,  or  other  irregular 
shapes,  and  is  rather  harder  and  heavier  than  calcite. 

DOLOMITE  assumes  a  rhombohedral  crystallisation,  and  is  a 
compound  of  54.4  of  magnesium-carbonate,  with  45.6  of  calcium- 
carbonate.  It  is  rather  harder  than  calcite,  and  does  not  effer- 
vesce so  freely  with  acid.  It  occurs  in  strings  and  veins  like 
calcite,  but  also  in  massive  beds  having  a  prevalent  pale  yellow 
or  brown  colour  (owing  to  hydrated  peroxide  of  iron),  a  granu- 
lar and  often  cavernous  texture,  and  a  tendency  to  crumble  down 
on  exposure. 


Fig.  60. —  Calcite  in  the  form  of  dog-tooth  spar. 

SIDERITE  (chalybite,  spathic  iron,  ferrous  carbonate,  FeCO), 
another  rhombohedral  carbonate,  contains  62  per  cent  of  ferrous 
oxide  or  protoxide  of  iron.  In  its  crystalline  form  it  is  gray  or 
brown,  becoming  much  darker  on  exposure  as  the  protoxide 
passes  into  peroxide.  It  also  occurs  mixed  with  clay  in  concre- 
tions and  beds,  frequently  associated  with  remains  of  plants  and 
animals  (Sphcerosiderite,  Clay-ironstone,  Figs.  61,  65). 

SULPHATES. —  Two  sulphates  deserve  notice  for  their  impor- 
tance among  rock-masses  —  those  of  lime  and  baryta, 

GYPSUM  (hydrous  calcium-sulphate,  CaSO^+SHOz)  occurs  in 
monoclinic  crystals,  commonly  with  the  form  of  right  rhomboidal 
prisms  (Fig.  62,  a),  which  not  infrequently  appear  as  macles  or 
twin-crystals  (Fig.  62,  b).  When  pure  it  is  clear  and  colour- 
less, with  a  peculiar  pearly  lustre  (Selenite)  ;  it  is  found  fibrous 


ELEMENTS  AND  MINERALS 


141 


with  a  silky  sheen  (Satin-spar),  also  white  and  granular  (Ala- 
baster). It  is  so  soft  as  to  be  easily  cut  with  a  knife  or  even 
scratched  with  the  finger-nails.  It  is  readily  distinguished  from 
calcite  by  its  crystalline  form,  softness,,  and  non-effervescence 
with  acid.  When  burnt  it  be- 
comes an  opaque  white  powder 
(plaster  of  Paris).  Gypsum 
occurs  in  beds  associated  with 
sheets  of  rock-salt  and  also  with 
dolomite;  it  is  soluble  in  water, 
and  is  found  in  many  springs  and 
rivers,  as  well  as  in  the  sea.  One 
thousand  parts  of  water  at  32° 
Fahr.  dissolve  2.05  parts  of  sul- 
phate of  lime;  but  the  solubility 
of  the  substance  is  increased  in 
the  presence  of  common  salt,  a 
thousand  parts  of  a  saturated 
solution  of  common  salt  taking  up 

as  much  as  8.2  parts  of  the  sul-  pig.       61. —  Sphserosiderite 
phate. 

Anhydrous  calcium-sulphate  or 
ANHYDRITE  is  harder  and  heavier  than  gypsum,  and  is  found 
extensively  in  beds  associated  with  rock-salt  deposits.  By 
absorbing  water,  it  increases  in  bulk  and  passes  into  gypsum. 

BARYTES  (Heavy  spar,  barium-sulphate,  BaSO*),  the  usual 
form  in  which  the  metal  barium  is  distributed  over  the  globe, 
crystallises  in  orthorhombic  prisms  which  are  generally  tabular; 
but  most  frequently  it  occurs  in  various  massive  forms.  The 
purer  varieties  are  transparent  or  translucent,  but  in  general  the 
mineral  is  dull  yellowish  or  pinkish  white,  with  a  vitreous  lustre, 
and  is  readily  recognisable  from  other  similar  substances  by  its 
great  weight;  it  does  not  effervesce  with  acids.  Barytes  is 
usually  met  with  in  veins  traversing  rocks,  especially  in  associa- 
tion with  metallic  ores. 

PHOSPHATES. —  Only  one  of  these  requires  -to  be  enumerated 
in  the  present  list  of  minerals  —  the  phosphate  of  lime  or 
Apatite. 

APATITE  (tricalcic  phosphate,  phosphate  of  lime)  crystallises 
in  hexagonal  prisms  which,  as  minute  colourless  needles,  are 


or 

Clay-ironstone     concretion     en- 
closing portion  of  a   fern. 


142 


GEOLOGY 


abundant  in  many  crystalline  rocks;  it  also  occurs  in  large 
crystals  and  in  amorphous  beds  associated  with  gneiss.  It  is 
soluble  in  water  containing  carbonic  acid,  ammoniacal  salts,  com- 
mon salt,  and  other  salts.  Hence  its  introduction  into  the  soil, 
and  its  absorption  by  plants,  as  already  mentioned. 


a  6 

Fig.  62. —  Gypsum  crystals. 


FLUORIDES. —  The  only  member  of  this  family  occurring  con- 
spicuously in  the  mineral  kingdom  is  calcium  fluoride  or  FLUOR- 
SPAR (Fluorite,  CaF*),  which,  in  the  form  of  colourless,  but 
more  commonly  light  green,  purple,  or  yellow  cubes,  is  found  in 


Fig.  63. —  Group  of  fluor-spar  crystals. 


mineral     veins     not     infrequently     accompanying     lead-ores 
(Fig.  63). 

CHLORIDES. —  Reference  has  already  been  made  to  the  only 
chloride  which  occurs  plentifully  as  a  rock-mass,  the  chloride  of 


ELEMENTS  AND  MINERALS 


143 


sodium,  known  as  HALITE  or  Hock-salt  (NaCl,  chlorine  60.64, 
sodium  39.36).  It  crystallises  in  cubical  forms,  and  is  also 
found  massive  in  beds  that  mark  the  evaporation  of  former  salt- 
lakes  or  inland  seas. 

SULPHIDES. —  Many  combinations  of  sulphur  with  the  metals 
occur,  some  of  them  of  great  commercial  value ;  but  the  only  one 
that  need  be  mentioned  here  for  its  wide  diffusion  as  a  rock- 
constituent  is  the  iron-disulphide  (FeS?),  in  which  the  elements 
are  combined  in  the  proportion  of  46.7  iron  and  53.3  sulphur. 


Fig.  64.—  Concretions, 
o,  6,  "  Fairy  stones ;  "  c,  Pyrite,  showing  internal  radiated  structure. 

This  substance  assumes  two  crystalline  forms:  (1)  PYRITE 
which  occurs  in  cubes  and  other  forms  of  the  first  or  monometric 
system,  of  a  bronze-yellow  colour  and  metallic  lustre,  so  hard  as 
to  strike  fire  with  steel,  and  giving  a  brownish-black  powder 
when  scratched.  This  mineral  is  abundantly  diffused  in  minute 
grains,  strings,  veins,  concretions  (Fig.  64,  c),  and  crystals  in 
many  different  kinds  of  rocks ;  it  is  usually  recognisable  by  its 
colour,  lustre,  and  hardness;  (2)  MARCASITE  (white  pyrite) 


144  GEOLOGY 

crystallises  in  the  tetragonal  system,  has  a  paler  colour  than 
ordinary  pyrite,  and  is  much  more  liable  to  decomposition. 
This  form,  rather  than  pyrite,  is  usually  associated  with  the 
remains  of  plants  and  animals  imbedded  among  rocks.  The 
sulphide  has  no  doubt  often  been  precipitated  round  decaying 
organisms  by  their  effect  in  reducing  sulphate  of  iron.  By  its 
ready  decomposition,  marcasite  gives  rise  to  the  production  of 
sulphuric  acid  and  the  consequent  formation  of  sulphates.  One 
of  the  most  frequent  indications  of  this  decomposition  is  the 
rise  of  chalybeate  springs. 


THE  MORE  IMPORTANT  ROCKS  145 


CHAPTEE  XI. 

THE  MORE   IMPORTANT  ROCKS. 

FEOM  the  distribution  of  the  more  important  elements  in 
the  earth's  crust  and  the  mineral  forms  which  they 
assume,,  we  have  now  to  advance  a  stage  farther  and  in- 
quire how  the  minerals  are  combined  and  distributed  so  as  to 
build  up  the  crust.  As  a  rule,  simple  minerals  do  not  occur 
alone  in  large  masses ;  more  usually  they  are  combined  in  various 
proportions  to  form  what  are  known  as  Eocks.  A  rock  may  be 
denned  as  a  mass  of  inorganic  matter,  composed  of  one  or  more 
minerals,  having  for  the  most  part  a  variable  chemical  composi- 
tion, with  no  necessarily  symmetrical  external  form,  and  rang- 
ing in  cohesion  from  loose  or  feebly  aggregated  debris  up  to 
the  most  solid  stone.  Blown  sand,  peat,  coal,  sandstone,  lime- 
stone, lava,  granite,  though  so  unlike  each  other,  are  all  included 
under  the  general  name  of  Eocks. 

In  entering  upon  the  study  of  rocks,  or  the  division  of  geology 
known  as  Petrography,  it  is  desirable  to  be  provided  with  such 
helps  as  are  needed  for  determining  leading  external  charac- 
ters; in  particular,  a  hammer  to  detach  fresh  splinters  of  rock, 
a  pocket-knife  for  trying  the  hardness  of  minerals,  a  small  phial 
of  dilute  hydrochloric  acid  for  detecting  carbonate  of  lime,  and 
a  pocket  lens.  The  learner,  however,  must  bear  in  mind  that 
the  thorough  investigation  of  rocks  is  a  laborious  pursuit,  requir- 
ing qualifications  in  chemistry  and  mineralogy.  He  must  not 
expect  to  be  able  to  recognise  rocks  from  description  until  he 
has  made  good  progress  in  the  study.  As  already  stated  on  a 
previous  page,  he  must  examine  the  objects  themselves,  and  for 
this  purpose  he  will  find  much  advantage  in  procuring  a  set  of 
named  specimens,  and  making  himself  familiar  with  such  of 
their  characters  as  he  can  himself  readily  observe. 

Great  light  has  in  recent  years  been  thrown  upon  the  struc- 
ture and  history  of  rocks  by  examining  them  with  the  micro- 


146  GEOLOGY 

scope.  For  this  purpose,  a  thin  chip  or  slice  of  the  rock  to  be 
studied  is  ground  smooth  with  emery  and  water,  and  after  being 
polished  with  flour-emery  upon  plate-glass,  the  polished  side  is 
cemented  with  Canada  balsam  to  a  piece  of  glass,  and  the 
other  side  is  then  ground  down  until  the  specimen  is  so  thin 
as  to  be  transparent.  Thin  sections  of  rock  thus  prepared 
(which  can  now  be  obtained  from  any  good  mineral-dealer)  reveal 
under  the  microscope  the  minutest  kinds  of  rock-structure.  Not 
only  can  the  component  minerals  be  detected,  but  it  is  often 
possible  to  tell  the  order  in  which  they  have  appeared,  and  what 
has  been  the  probable  origin  and  history  of  the  rock.  Some 
illustrations  of  this  method  of  investigation  will  be  given  in  a 
later  part  of  the  present  chapter.  It  will  be  of  advantage  to 
begin  by  taking  note  of  some  of  the  more  important  characters 
of  rocks,,  and  of  the  names  which  geologists  apply  to  them. 


Fig.  65. —  Section  of  a  septarian  nodule,  with  coprolite  of  a  fish  as  a 

nucleus. 

Some  important  Terms  applied  to  Rocks. 

Sedimentary  —  composed  of  sediment  which  may  be  either  a 
mechanically  suspended  detritus,  such  as  mud,  sand,  shells,  or 
gravel;  or  a  chemical  precipitate,  as  rock-salt  and  calcareous 
tufa.  The  various  deposits  which  are  accumulated  on  the  floors 
of  lakes,  in  river-courses,  and  on  the  bed  of  the  sea,  axe  ex- 
amples of  sedimentary  rocks. 

Fragmental,  Clastic  —  composed  of  fragments  derived  from 
some  previous  rock.  All  ordinary  detritus  is  of  this  nature. 


THE  MORE  IMPORTANT  ROCKS  147 

Concretionary  —  composed  of  mineral  matter  which  has  been 
aggregated  round  some  centre  so  as  to  form  rounded  or  irregu- 
larly-shaped lumps.  Some  minerals,  particularly  pyrite  (Fig. 
64  c),  marcasite,  siderite,  and  calcite,  are  frequently  found  in 


Fig.  66.— Piece  of  oolite. 

concretionary  forms,  especially  round  some  organic  relic,  such  as 
a  shell  or  plant  (Figs.  61,  65).  In  alluvial  clay,  calcareous  con- 
cretions which  often  take  curious  imitative  shapes,  are  known  as 
"fairy  stones "  (Fig.  64,  a,  I). 


Fig.  67.— Piece  of  pisolite. 

When  nodules  of  limestone,  ironstone,  or  cement-stone  are 
marked  internally  by  cracks  which  radiate  towards,  but  do  not 
reach,  the  outside,  and  are  filled  up  with  calcite  or  other  mineral, 
they  are  known  as  Septaria  or  septarian  nodules  (Fig.  65,  and 
layer  13  in  Fig.  80). 


148  GEOLOGY 

Oolitic  —  made  up  of  spherical  grains,  each  of  which  has 
been  formed  by  the  deposition  of  successive  coatings  of  mineral 
matter  round  some  grain  of  sand,  fragment  of  shell,  or  other 
foreign  particle  (Fig.  66).  A  rock  with  this  structure  looks 
like  fish-roe,  hence  the  name  oolite  or  roe-stone;  but  when  the 
granules  are  like  peas,  the  rock  becomes  pisolitic  (pea-stone, 
Fig.  67).  This  peculiar  structure  is  produced  in  water  (springs, 
lakes,  or  enclosed  parts  of  the  sea),  wherein  dissolved  mineral 
matter  (usually  carbonate  of  lime)  is  so  abundant  as  to  be  de- 
posited in  thin  pellicles  round  the  grains  of  sediment  that  are 
kept  in  motion  by  the  current. 

Stratified,  Bedded  —  arranged  in  layers,  strata,  or  beds  lying 
generally  parallel  to  each  other,  as  in  ordinary  sedimentary 
deposits  (Fig.  79). 

Aqueous  —  laid  down  in  water,  comprising  nearly  the  whole 
of  the  sedimentary  and  stratified  rocks. 

Unstratified,,  Massive  —  having  no  arrangement  in  definite 
layers  or  strata.  Lavas  and  the  other  eruptive  rocks  are  exam- 
ples (Chapter  XIV). 

Eruptive,  Igneous  —  forced  upwards  in  a  molten  or  plastic 
condition  into  or  through  the  earth's  crust.  All  lavas  are  Erup- 
tive or  Igneous  rocks,  also  called  Volcanic  because  erupted  to  the 
surface  by  volcanoes.  In  the  same  division  must  be  classed 
granite  and  allied  masses,  which  have  been  thrust  through  rocks 
at  some  depth  within  the  earth's  crust  and  may  not  have  been 
directly  connected  with  any  volcanic  eruption;  such  rocks  are 
sometimes  called  Plutonic  or  Hypogene. 

Crystalline  —  consisting  wholly  or  chiefly  of  crystals  or  crys- 
talline grains.  Eocks  of  this  nature  may  have  arisen  from  (a) 
igneous  fusion,  as  in  the  case  of  lavas,  where  the  minerals  have 
separated  out  of  a  molten  glass,  or  what  is  called  a  Magma;  (b) 
aqueous  solution,  as  where  crystalline  calcite  forms  stalactite 
and  stalagmite  in  a  cavern;  (c)  sublimation,  where  the  materials 
have  crystallised  out  of  hot  vapours,  as  in  the  vents  and  clefts  of 
volcanoes. 

By  the  aid  of  the  microscope  many  rocks  which  to  the  naked 
eye  show  no  definite  structure  can  be  shown  to  be  wholly  or 
partially  crystalline.  Moreover,  it  can  often  be  ascertained  that 
the  crystals  or  crystalline  grains  in  a  rock,  as  they  were  crystal- 
lising out  of  their  solution,  have  enclosed  various  foreign  bodies. 


THE  MORE  IMPORTANT  ROCKS 


149 


Among  the  objects  thus  taken  up  are  minute  globules  of  gas, 
which  are  prodigiously  abundant  in  certain  minerals  in  some 
lavas  ;  liquids,  usually  water,  enclosed  in  cavities  of  the  crystals, 
but  not  quite  filling  them,  and  leaving  a  minute  freely-moving 
bubble   (Fig.  68)  ;  glass,  filling  globular 
spaces,  probably  part  of  the  original  glassy 
magma  of  the  rock;  crystals  and  crystal- 
lites (rudimentary  crystalline  forms,  Fig. 
69)    of  other  minerals.     Thus  a  crystal, 
which  to  the  eye  may  appear  quite  free 
from  impurities,  may  be  found  to  be  full 
of  various  kinds  of  enclosures.     Obvious- 
ly the  study  of  these  enclosures  cannot 
Pig.    68.—  Cavities    in  but  throw  light  on  the  conditions  under 
quartz  containing  liq-  which  the  rocks  enclosing  them  were  pro- 
uids   (magnified). 


There  are  various  types  of  crystalline  structure  which  can  best 
be  examined  under  the  microscope,  as  Holo  crystalline,  composed 
entirely  of  crystalline  elements  without  any  interstitial  glass  — 
one  of  the  most  characteristic  types  of  this  structure  is  found  in 
granite,  hence  it  is  sometimes  termed  the  granitic  or  granitoid 
structure;  Semi-crystalline,  consisting  partly  of  crystals,  but 
with  a  ground  mass  or  base  which  may  be  partly  glassy  or  vari- 


Fig.  69. —  Various  forms  of  crystallites   (highly  magnified). 

ously   devitrified;   Felsitic   or  micro felsitic  —  composed  of   in- 
definite half -effaced  granules  and  filaments. 

Glassy,  Vitreous  —  having  a  structure  and  aspect  like  that  of 
artificial  glass.  Some  lavas,  obsidian  for  example,  have  solidified 
as  natural  glasses,  and  look  not  unlike  masses  of  dark  bottle- 
glass.  In  almost  all  cases,  however,  they  contain  dispersed 
crystals,  crystallites,  or  other  enclosures.  These  substances  have 
generally  multiplied  to  such  an  extent  in  most  lavas  as  to  leave 


150  GEOLOGY 

only  small  interstitial  portions  of  the  original  glass,  while  in 
many  cases  the  glass  has  entirely  disappeared.  When  a  glass  has 
thus  been  converted  into  a  dull,  opaque,  stony,  or  lithoid  mass,  or 
into  a  completely  crystalline  substance,  it  is  said  to  be  devitri- 
fied.  The  microscope  enables  us  to  prove  many  crystalline 
eruptive  rocks  to  have  been  once  molten  glass  which  by  a  process 
of  devitrification  have  been  brought  into  their  present  more  or 
less  crystalline  condition. 

Porphyritic  —  composed  of  a  compact  or  crystalline  base  or 
matrix,  through  which  are  scattered  conspicuous  crystals  much 
larger  than  those  of  the  base,  and  generally  of  some  felspar.  Many 
eruptive  rocks  have  this  structure  and  are  sometimes  spoken 
of  as  "  porphyries."  The  large  crystals  existed  in  the  rock  while 


Fig.  70. —  Porphyritic  structure. 

still  in  a  mobile  state  within  the  earth's  crust,  while  the  minuter 
crystals  of  the  base  were  developed  by  a  later  process  of  crystal- 
lisation during  the  consolidation  of  the  rock.  In  the  successive 
zones  of  growth  which  porphyritic  crystals  often  present,  we  may 
note  by  the  enclosed  minerals  some  of  the  successive  stages  of 
consolidation. 

Spherulitic  —  composed  of  or  containing  small  pea-like  globu- 
lar bodies  (Spfierulites)  which  show  a  minutely  fibrous  internal 
structure  radiating  from  the  centre  (Fig.  71,  A).  This  struc- 
ture is  particularly  observable  in  vitreous  rocks,  where  it  appears 
to  be  one  of  the  stages  of  devitrification. 

Perlitic. —  Many  vitreous  rocks  show  a  minute  fissured  struc- 
ture as  one  of  the  accompaniments  of  devitrification.  In  the 


THE  MORE  IMPORTANT  ROCKS  151 

structure  Termed  perlitic  the  original  glass  has  had  a  series  of 
reticulated  and  globular  or  spiral  cracks  developed  in  it,  some- 
times giving  rise  to  globules  composed  of  successive  thin  shells. 
Vesicular,  Cellular  —  containing  spheroidal  or  irregularly 
shaped  cavities.  In  many  eruptive  rocks  (as  in  modern  lavas) 
the  expansion  of  interstitial  steam,  while  the  mass  was  still  in  a 
molten  condition,  has  produced  this  cellular  structure  (Fig.  35), 
the  vesicles  have  usually  remarkably  smooth  walls;  they  may 
form  a  comparatively  small  part  of  the  whole  mass,,  or  they  may 
so  increase  as  to  make  pieces  of  the  rock  capable  of  floating  on 
water.  Where  the  vesicular  structure  is  conjoined  with  more 
solid  parts,  as  in  the  irregular  slags  of  an  iron  furnace,  it  may  be 
called  slaggy.  Where,  as  in  the  scoria?  of  a  volcano,  the  cellular 
and  solid  parts  are  in  about  equal  proportions,  and  the  vesicles 


A 

Fig.  71. —  Spherulites  and  fluxion-structure.  A,  Spherulites,  as  seen 
under  the  microscope  (with  polarised  light).  B,  Fluxion-structure  of 
Obsidian,  as  seen  under  the  microscope. 

vary  greatly  in  numbers  and  size  within  short  distances,  the 
structure  may  be  termed  scoriaceous.  The  lighter  and  more 
froth-like  varieties  that  can  float  on  water  are  said  to  be  pumi- 
ceous,  or  to  have  the  characters  of  pumice.  When  exposed  to 
the  influence  of  percolating  water,  vesicular  rocks  have  had  their 
vesicles  filled  up  by  the  deposition  of  various  minerals  from 
solution,  especially  quartz,  calcite,  and  zeolites.  These  substances 
first  begin  to  encrust  the  walls  of  the  cells,  and  as  layer  succeeds 
layer  they  gradually  fill  the  cells  up  (Fig.  52)  ;  as  the  cells  have 
not  infrequently  been  elongated  in  one  direction  by  the  motion  of 
the  rock  before  consolidation  was  completed  (Fig.  37),  the 
mineral  deposits  in  them,  taking  their  exact  moulds,  appear  as 
oval  or  almond-shaped  bodies.  Hence  rocks  which  have  been 


152  GEOLOGY 

treated  in  this  way  are  called  Amygdaloids,  and  the  kernels 
filling  up  the  cells  are  known  as  Amygdules  (Fig.  35).  An 
amygdaloidal  rock,  therefore,  was  originally  a  molten  lava,  ren- 
dered cellular  by  the  expansion  of  its  absorbed  steam  and  gases, 
its  vesicles  having  been  subsequently  filled  up  by  the  deposit  in 
them  of  mineral  matter,  often  derived  out  of  the  surrounding 
rock  by  the  decomposing  and  rearranging  action  of  percolating 
water. 

Flow-structure,  Fluxion-structure  —  an  arrangement  of  the 
crystallites,  crystals,  or  particles  of  a  rock  in  streaky  lines,  the 
minuter  forms  being  grouped  round  the  larger,  indicative  of  the 
internal  movement  of  the  mass  previous  to  its  consolidation. 
The  lines  are  those  in  which  the  particles  flowed  past  each  other, 


Fig.  72. —  Schistose  structure. 

the  larger  crystals  giving  rise  to  obstructions  and  eddies  in  the 
movement  of  the  smaller  objects  past  them.  This  structure  is 
characteristic  of  many  once  molten  rocks;  it  is  well  seen  in 
obsidian  (Fig.  71,  B).  But  it  is  also  found  in  rocks  which,  by 
enormous  stresses  within  the  earth's  crust,  have  been  crushed  and 
made  to  undergo  an  interstitial  movement  like  that  of  the  flow  of 
liquids.  The  most  solid  gneisses  and  granites  have  in  this  way 
been  so  sheared  and  squeezed  that  their  component  minerals 
have  been  crushed  into  a  fine  compact  mass,  through  which  the 
streaking  lines  of  flow  are  sometimes  displayed  with  singular 
clearness. 

Mylonitic  —  a  name  sometimes  applied  to  rocks  which  by 
terrestrial  movement  have  had  their  original  structure  entirely 


CLASSIFICATION  OF  ROCKS  153 

obliterated,  and  which  now  present  only  a  dull,  crushed  felsitic 
mass,  sometimes  partially  or  completely  recrystallised. 

Schistose,  Foliated  —  consisting  of  minerals  that  have  crystal- 
lised in  approximately  parallel,  wavy,  and  irregular  laminae,  lay- 
ers or  folia  (Fig.  72).  Such  rocks  are  called  generally  schists. 
They  have,  in  large  measure,  been  formed  by  the  alteration  or 
metamorphism  of  other  rocks  of  various  kinds  by  the  vast  ter- 
restrial movements  referred  to  in  the  foregoing  paragraphs  (see 
Chapter  XIII). 

CLASSIFICATION  OF  ROCKS. 

Various  schemes  of  classification  of  rocks  are  in  use  among 
geologists,  some  based  on  mode  of  origin,  others  on  mineral 
composition  or  structure.  For  the  purpose  of  the  learner,  per- 
haps the  most  instructive  and  useful  arrangement  is  one  which 
as  far  as  possible  combines  the  advantages  of  both  these  systems. 
Accordingly,  in  the  following  account  of  the  more  important 
rocks  which  enter  into  the  structure  of  the  earth's  crust,  a  three- 
fold subdivision  will  be  adopted  into:  (i)  sedimentary  rocks; 
(ii)  eruptive  rocks;  (iii)  schistose  rocks. 

I.  SEDIMENTARY  BOCKS. 

This  division  includes  the  largest  number,  and  to  the  geologist 
the  most  important  of  the  rocks  accessible  to  our  notice.  It 
comprises  the  various  deposits  that  arise  from  the  decay  of  the 
surface  of  the  land  and  are  laid  down  on  the  land  or  over  the 
bed  of  the  sea,  together  with  all  those  directly  or  indirectly  due 
to  the  growth  of  plants  and  animals.  It  thus  embraces  those 
which  constitute  the  main  mass  of  the  earth's  crust  so  far  as 
known  to  us,  and  which  contain  the  evidence  whence  the  geo- 
logical history  of  the  earth  is  chiefly  worked  out.  It  is,  therefore, 
worthy  of  the  earliest  and  closest  "attention  of  the  student. 

Sedimentary  rocks,  being  due  to  the  deposition  of  some  kind 
of  sediment  or  detritus,  are  obviously  not  original  or  primitive 
rocks.  They  have  all  been  derived  from  some  source,  the  nature 
of  which,  if  not  its  actual  site,  can  usually  be  determined.  In 
no  case,  therefore,  can  sedimentary  rocks  carry  us  back  to  the 
beginning  of  things;  they  are  themselves  derivative  and  pre- 


154  GEOLOGY 

suppose  the  existence  of  some  older  rock  or  material  from  which 
they  could  be  derived. 

One  of  their  most  obvious  characters  is  that,  as  a  rule,  they 
are  stratified.  They  have  been  deposited,  usually  in  water,  some- 
times in  air,  layer  above  layer,  and  bed  above  bed,  each  of  these 
strata  marking  a  particular  interval  in  the  progress  of  deposition 
(Chapter  XII).  As  regards  their  mode  of  origin,  they  may  be 
subdivided  into  three  great  sections:  (1)  fragmental  of  clastic, 
composed  of  fragments  of  pre-existing  rocks;  (2)  chemically  pre- 
cipitated, as  in  the  deposits  from  mineral  springs;  and  (3) 
formed  of  the  remains  of  organisms,  as  in  peat  and  coral-rock. 


Fig.   73. —  Brecciated  structure  —  volcanic  breccia,  a  rock   composed  of 
angular  fragments' of  lava,  in  a  paste  of  finer  volcanic  debris. 

(1)  Fragmental  or  Clastic  Rocks. 

These  are  masses  of  mechanically-formed  sediment,  derived 
from  the  destruction  of  older  rocks;  they  vary  in  coherence 
from  loose  sand  or  mud  up  to  the  most  compact  sandstone  or 
conglomerate;  they  are  accumulating  abundantly  at  the  present 
time  in  the  beds  of  rivers  and  lakes,  and  on  the  floor  of  the 
sea,  and  they  have  been  formed  in  a  similar  way  all  over  the 
globe  from  the  earliest  periods  of  known  geological  history. 
Some  of  the  more  frequent  kinds  are  the  following: — 

CLIFF-DEBRIS  —  coarse  angular  rubbish,  including  large 
blocks  of  stone,  disengaged  by  the  weather  from  cliffs  and 
other  bare  faces  of  rock.  This  kind  of  detritus  is  formed  abun- 
dantly in  rugged  and  mountainous  regions,  especially  where  the 
action  of  frost  is  severe ;  it  slides  down  the  slopes  and  accumu- 


CLASSIFICATION  OF  ROCKS  155 

lates  at  their  foot,  unless  washed  away  by  torrents.  In 
glacier- valleys  it  descends  to,  the  ice,  where,  gathering  into 
moraines  (Chapter  VI),  it  is  transported  to  lower  levels.  The 
perched  blocks  of  such  valleys  are  some  of  the  larger  fragments 
of  this  cliff-debris  left  stranded  by  the  ice,  and  from  around 
which  the  smaller  detritus  has  been  washed  away  (Fig.  23). 

SOIL,  SUBSOIL,  described  in  Chapter  II,  represent  the  result 
of  the  subaerial  decomposition  of  the  surface  of  the  land. 

BRECCIA  —  a  rock  composed  of  angular  fragments.  .  Such  a 
rock  shows  that  its  materials  have  not  travelled  far;  otherwise, 
they  would  have  lost  their  edges,  and  would  have  been  more 
or  less  rounded.  Ordinary  cliff-debris  may  consolidate  into  a 
breccia,  more  especially  where  it  falls  into  water  and  is  allowed 


Fig.   74. —  Conglomerate. 

to  gather  on  the  bottom.  The  angular  fragments  shot  out  of  a 
volcano  often  accumulate  into  volcanic  breccia  (Fig.  73).  A 
rock  with  abundant  angular  fragments  is  said  to  be  brecciated. 
GRAVEL  —  loose  rounded  water-worn  detritus,  in  which  the 
pebbles  range  in  average  size  between  that  of  a  small  pea  and 
that  of  a  walnut;  where  they  are  larger  they  form  Shingle. 
They  may  consist  of  fragments  of  any  kind  of  rock,  though 
having  resulted  from  more  or  less  violent  water-action,  as  a 
rule,  pieces  of  only  the  more  durable  stones  are  found  in  them. 
Quartz  and  other  siliceous  materials,  from  their  great  hardness, 
are  better  able  to  withstand  the  grinding  to  which  the  detritus 
on  an  exposed  sea-shore,  or  in  the  bed  of  a  rapid  stream,  is 
subjected.  Hence  quartzose  and  siliceous  pebbles  are  the  most 
frequent  constituents  of  gravel  and  shingle. 


156  GEOLOGY 

CONGLOMERATE  —  a  name  given  to  gravel  and  shingle  when 
they  have  been  consolidated  into  stone,  the  pebbles  being  bound 
together  by  some  kind  of  paste  or  cementing  material,  which 
may  be  fine  hardened  sand,  clay,  or  some  calcareous,  siliceous, 
or  ferruginous  cement  (Fig.  74).  As  above  remarked  with 
regard  to  gravel,  the  component  materials  of  conglomerate  may 
have  been  derived  from  any  kind  of  rock,  but  siliceous  pebbles 
are  of  most  common  occurrence.  Different  names  are  given  to 
conglomerates,  according  to  the  nature  of  the  pebbles,  as  quartz- 
conglomerate,  flint-conglomerate,  limestone-conglomerate. 

SAND  —  a  name  given  to  fine  kinds  of  detritus,  the  grains  of 
which  may  vary  from  the  size  of  a  small  pea 'down  to  minute 
particles  that  can  only  be  detected  with  a  lens.  In  general,  for 
the  reason  already  assigned  in  the  case  of  gravel,  the  component 
grains  of  sand  are  of  quartz  or  of  some  other  durable  material. 
Examined  with  a  good  magnifying  glass,  they  are  seen  to  be 
usually  rounded,  water-worn,  but  sometimes  angular,  unworn 
particles  of  indefinite  shapes  which,  except  in  their  smaller  size, 
resemble  those  of  gravel-stones.  Sand  may  be  formed  by  the 
disintegration  of  the  surface  of  rocks  exposed  to  the  weather, 
more  especially  in  dry  climates,  where  there  is  a  great  difference 
between  the  temperature  of  the  day  and  the  night.  The 
loosened  particles  are  blown  away  by  the  wind,  and  may  be 
heaped  up  into  great  sand-wastes,  as  in  the  tracts  known  as 
Deserts.  On  a  sea-coast,  where  a  sandy  beach  is  liable  to  be 
laid  bare  and  exposed  to  be  dried  between  tides  by  breezes  blow- 
ing from  the  sea,  the  upper  particles  of  sand  are  lifted  up  by 
the  wind  and  borne  away  landward,  to  be  piled  up  into  dunes. 
In  some  places  the  materials  of  sand  are  derived  mainly 
from  the  remains  of  calcareous  sea-weeds,  shells,  corallines,  and 
other  calcareous  organisms  exposed  to  the  pounding  action  of 
the"  surf.  A  sand  composed  of  such  materials  speedily  hardens 
into  a  more  or  less  coherent  and  even  compact  limestone,  for 
rain  falling  on  it  dissolves  some  carbonate  of  lime  which,  being 
immediately  deposited  again,  as  the  moisture  evaporates,  coats 
the  grains  of  sand  and  cements  them  together.  At  Bermuda, 
as  already  stated,  all  the  rock  above  sea-level  has  been  formed 
in  this  way,  and  some  of  it  is  hard  enough  to  make  a  good 
stone  for  building.  The  ordinary  siliceous  or  quartzose  sand 


CLASSIFICATION  OF  ROCKS  157 

remains  loose,  unless  its  grains  are  made  to  cohere  by  some 
kind  of  cement,  when  it  becomes  sandstone. 

SANDSTONE  —  consolidated  sand.  The  grains  are  chiefly 
quartz,  but  may  include  particles  of  any  other  mineral  or  rock; 
they  are  bound  together  by  some  kind  of  cement  which  has 
either  been  laid  down  with  them  at  the  time  of  their  deposi- 
tion, or  has  subsequently  been  introduced  by  water  permeating 
the  sand.  The  cementing  material  may  be  argillaceous  —  that 
is,  some  kind  of  clay;  or  calcareous •,  consisting  of  carbonate  of 
lime;  or  ferruginous,  composed  mainly  of  peroxide  of  iron;  or 
siliceous,  where  silica  has  been  deposited  in  the  interstices  of 
the  mass.  The  colours  of  sandstone  vary  chiefly  with  the 
nature  of  this  cementing  material.  The  hydrous  peroxide  of  iron 
colours  them  shades  of  yellow  and  brown ;  the  anhydrous  peroxide 
of  iron  gives  them  different  hues  of  red;  the  mineral  glauconite 
tints  them  a  greenish  hue.  Some  varieties  of  sandstone  are 
named  after  a  conspicuous  component  or  structure;  thus 
micaceous  sandstone  is  distinguished  by  abundant  spangles  of 
mica  deposited  along  the  bedding  planes,  whereby  the  rock  can 
be  split  up  into  thin  layers;  freestone  —  a  thick-bedded  sand- 
stone that  does  not  tend  to  split  up  in  any  one  direction,  and 
can  therefore  be  cut  into  blocks  of  any  size  and  form;  glau- 
conitic  sandstone  (green  sand),  containing  green  grains  and 
kernels  of  glauconite;  quartzose  sandstone,  conspicuously  com- 
posed of  quartz-grains;  grit  —  a  sandstone  formed  of  coarse  or 
sharp,  somewhat  angular  grains  of  quartz. 

GRAYWACKE  —  a  grayish,  compact,  granular  rock,  composed 
of  rounded  or  subangular  grains  of  quartz  and  other  minerals 
or  rocks,  cemented  together  in  a  compact  paste;  it  differs  from 
sandstone  chiefly  in  its  darker  colour,  in  the  proportion  of 
other  grains  than  those  of  quartz,  and  in  the  presence  of  a 
tough  cement. 

The  rocks  above  enumerated  represent  the  coarser  and  more 
durable  kinds  of  detritus  derived  from  the  weathering  of  the 
surface  of  the  land ;  but  during  the  progress  of  the  decomposi- 
tion from  which  these  materials  are  derived  some  of  the  com- 
ponent ingredients  of  the  rocks  decay  into  clay,  or  what  is 
called  argillaceous  sediment.  This  more  particularly  occurs  in 
the  case  of  felspars  and  other  aluminous  silicates,  the  decom- 
position of  which  produces  minute  particles  capable  of  being 


158  GEOLOGY 

lifted  up  and  carried  a  great  distance  by  running  water.  Hence 
argillaceous  sediment,  being  commonly  finer  in  grain,  travels 
farther,  on  the  whole,  than  quartzose  sediment;  and  beds  of 
clay  denote,  generally,  deeper  and  stiller  water  than  beds  of 
sand. 

CLAY  —  a  fine-grained  argillaceous  substance,  derived  from 
the  decay  and  hydration  of  aluminous  silicates,  white  when 
pure,  but  usually  mixed  with  impurities,  which  impart  to  it 
various  shades  of  gray,  green,  brown,  red,  purple,  or  blue;  it 
usually  contains  interstitial  water,  and  when  wet  can  be  kneaded 
between  the  fingers ;  when  dry  it  is  soft  and  friable,  and  adheres 
to  the  tongue.  Shaken  with  water  it  becomes  Mud;  even  a 
small  quantity  will  make  a  glass  of  water  turbid,  so  fine  are 
the  particles  of  which  it  is  composed. 

KAOLIN  —  the  name  given  to  the  white  purer  forms  of  clay, 
resulting  from  the  decomposition  of  the  felspars  of  granite  or 
similar  rocks;  it  is  sometimes  called  China-clay,  from  its  use 
in  the  manufacture  of  porcelain. 

FIRE-CLAY  —  a  white,  gray,  yellow,  or  black  clay,  nearly  free 
from  alkalies  and  iron,  and  capable  of  standing  a  great  heat 
without  fusing;  it  is  abundantly  found  underneath  coal-seams, 
where  it  represents  the  ancient  soil  on  which  the  plants  grew 
that  have  been  converted  into  coal. 

BRICK-CLAY  —  a  name  commonly  applied  to  any  clay,  loam, 
or  earth  from  which  bricks  can  be  made.  Such  deposits  are 
always  more  or  less  sandy  and  impure  clays;  in  the  south  of 
England  they  have  largely  arisen  from  the  prolonged  sub  aerial 
waste  of  the  Cretaceous  and  Tertiary  formations. 

MUDSTONE  —  a  compact  solidified  clay  or  clay-rock,  having 
little  or  no  tendency  to  split  into  thin  laminae. 

SHALE  —  clay  that  has  become  hard  and  splits  into  thin 
laminae  which  lie  parallel  with  the  planes  of  deposit  (Fig.  79). 
A  thoroughly  fissile  shale  can  be  subdivided  into  leaves  as  thin 
as  fine  cardboard.  This  is  the  common  form  which  the  clays 
of  the  older  geological  formations  have  assumed.  Gradations 
can  be  traced  from  shale  into  other  sedimentary  rocks;  thus, 
by  additions  of  sand  into  fissile  sandstones,  of  calcareous  cement 
into  limestone,  of  carbonate  of  iron  into  ironstone,  of  car- 
bonaceous matter  into  coal.  These  passages  are  interesting  as 
indications  of  the  conditions  under  which  the  rocks  were  formed. 


CLASSIFICATION  OF  ROCKS  159 

Where,  for  example,  shale  shades  off  into  coral-limestone,  we 
see  that  mud  gathered  over  one  part  of  the  sea-floor,  while 
not  far  off,  probably  in  clearer  water,  corals  flourished  and 
built  up  a  limestone  out  of  their  remains. 

LOESS  —  a  pale  somewhat  calcareous  and  sandy  clay,  found 
in  regions  where  it  has  probably  been  accumulated  by  the  drift- 
ing action  of  the  wind.  It  is  sufficiently  coherent  to  be  capable 
of  excavation  into  tunnels  and  passages,  and  in  China  is  even 
dug  out  into  houses  and  subterranean  villages.  It  occupies  parts 
of  the  valleys  of  the  Ehine,  Danube,  Mississippi,  and  other 
large  rivers,  but  also  crosses  watersheds. 

Fragmental  rocks  of  volcanic  origin  may  be  enumerated 
here.  They  consist  partly  of  materials  ejected  in  fragmentary 
form  from  volcanic  vents,  and  partly  of  the  detritus  derived 
from  the  disintegration  of  volcanic  rocks  already  erupted  to 
the  surface.  They  are  comprised  under  the  general  name  of 
Tuff. 

BOMBS  —  round  elliptical  or  discoidal  pieces  of  lava  which 
have  been  ejected  in  a  molten  state  from  an  active  vent,  and 
have  acquired  their  form  from  rapid  rotation  in  the  air  during 
ascent  and  descent.  They  are  often  very  cellular  or  even  quite 
empty  inside.  Where  the  large  ejected  stones  are  of  irregular 
forms,  and  appear  to  have  been  thrown  out  in  an  already  solidi- 
fied condition,  as  from  the  consolidated  crust  of  the  lava-plug, 
or  from  the  sides  of  the  funnel  or  crater,  they  are  called 
Volcanic  Blocks. 

LAPILLI  —  ejected  pieces  of  lava,  usually  vesicular  or  porous, 
from  the  size  of  a  pea  to  a  walnut  (Fig.  73). 

VOLCANIC  ASH  — ;  the  fine  dust  produced  by  the  explosion  of 
the  superheated  steam  absorbed  in  molten  lava.  Under  the 
microscope,  it  is  often  found  to  consist  of  minute  grains  of 
glass,  and  in  such  cases,  shows  that  the  lava  from  which  it  was 
derived,  rose  from  below  in  the  condition  of  a  liquid  glassy 
magma.  In  other  instances,  it  is  made  up  of  the  crystallites 
and  crystals  that  arose  during  the  devitrification  of  the  glass. 
It  consolidates  into  a  more  or  less  coherent  mass,  which  is 
known  as  Tuff,  and  which  may  receive  some  distinctive  name 
according  to  the  nature  of  the  lava  that  has  supplied  it,  as 
Basalt-tuff  and  Trachyte-tuff.  Most  tuffs  contain  angular  and 


160  GEOLOGY 

vesicular  pieces  of  lava,  and  sometimes  pass  into  coarse  breccias 
(Volcanic  Breccia).  In  many  cases,  they  enclose  the  remains 
of  plants  and  animals  which,  if  of  terrestrial  kinds,  indicate 
that  the  eruptions  took  place  on  land;  if  of  marine  species, 
that  the  volcanoes  were  probably  submarine. 

AGGLOMERATE  —  a  coarse,  usually  unstratified  accumulation 
of  blocks  of  lava  and  other  rocks,  not  infrequently  filling  up 
the  chimney  or  neck  of  a  volcanic  vent. 

(2)  Rocks  formed  by  Chemical  Precipitation. 

In  Chapter  V  it  was  pointed  out  that  all  natural  waters 
contain  in  solution  invisible  mineral  matter  which  they  have 
dissolved  out  of  the  rocks  of  the  earth's  crust,  and  that  the 
quantity  of  this  material  is  sometimes  so  great  that  it  is  pre- 
cipitated into  visible  form  as  the  water  evaporates.  The  sub- 
stance most  abundantly  dissolved  and  deposited  is  Carbonate 
of  lime.  Others  of  frequent  occurrence  are  Sulphate  of  lime. 
Chloride  of  sodium,  Silica,  Carbonate  of  magnesia,  and  various 
salts  of  iron.  Among  the  rocks  of  the  earth's  crust,  considerable 
masses  of  these  substances  have  been  piled  up  by  chemical 
precipitation. 

LIMESTONE  —  compact  or  crystalline  calcium-carbonate  (car- 
bonate of  lime)  which  may  be  nearly  pure,  or  may  contain 
sand,  clay,  or  other  impurity,  and  may  consequently  pass  into 
sandstone,  shale,  or  other  sedimentary  rock.  Probably  the  great 
majority  of  the  limestones  in  the  earth's  crust  have  been  formed 
by  the  agency  of  animals,  as  more  particularly  referred  to  at 
p.  164.  We  are  here  concerned  only  with  those  which  have 
been  deposited  from  chemical  solution.  The  most  familiar 
example  of  this  kind  of  limestone  is  afforded  by  stalactites 
and  stalagmite,  which  have  already  been  described  (Chapter  V 
and  Fig.  20).  Large  masses  of  it  have  been  deposited  by 
calcareous  springs  and  also  by  streams.  At  first,  it  is  a  fine 
white  milky  precipitate,  but  gradually  crystals  of  calcite  shape 
themselves  and  grow  out  of  it,  with  their  vertical  axes  usually 
at  right  angles  to  the  surface  of  deposit.  In  a  vertical  stalactite, 
consequently,  the  prisms  radiate  horizontally  from  the  centre 
outwards;  on  a  horizontal  surface  of  stalagmite  they  diverge 
perpendicular  to  the  floor.  A  mass  of  limestone,  not  originally 


CLASSIFICATION  OF  ROCKS  161 

crystalline,  may  thus  acquire  a  thoroughly  crystalline  internal 
structure  by  the  action  of  infiltrating  water  in  dissolving  the 
carbonate  of  lime  and  redepositing  it  in  a  crystalline  condition. 

Limestones  vary  greatly  in  texture  and  purity.  Some  are 
snow-white  and  distinctly  crystalline;  others  are  gray,  blue, 
yellow,  or  brown,  dull  and  compact,  and  full  of  various  im- 
purities. They  may  usually  be  detected  by  the  ease  with  which 
they  can  be  scratched,  and  their  copious  effervescence  when  a 
drop  of  weak  acid  is  put  on  the  scratched  surface.  Pure  lime- 
stone dissolves  entirely  in  hydrochloric  acid,  so  that  the  amount 
of  residue  is  an  indication  of  the  proportion  of  insoluble  im- 
purity. Among  the  varieties  of  limestone  the  following  may 
be  named: — Oolite,  a  limestone  composed  of  minute  spherical 
grains  like  the  roe  of  a  fish,  each  grain  being  composed  of 
concentrically  deposited  layers  or  shells  of  calcite  (Fig.  66)  ; 
Pisolite,  a  similar  rock,  where  the  grains  are  as  large  as  peas 
(Fig.  67)  ;  Travertine  or  calcareous  tufa,  a  white  porous 
crumbling  rock  which,  by  infiltration  of  carbonate  of  lime, 
may  acquire  a  compact  texture,  and  become  suitable  for  use  as 
building  stone;  Hydraulic  limestone,  containing  10  to  30 
per  cent  of  fine  sand  or  clay,  and  having  the  property,  after 
being  burnt,  of  hardening  under  water  into  a  firm  compact 
mortar. 

DOLOMITE,  MAGNESIAS  LIMESTOKE  —  this  substance  has  been 
already  referred  to  as  a  mineral  (p.  140)  ;  but  it  also  occurs 
in  large  masses  as  a  white  or  yellowish  crystalline  or  compact 
rock.  The  white  varieties  look  like  marble.  The  yellow  and 
brown  kinds  contain  various  impurities,  and  are  coloured  by 
iron-oxide.  Dolomite  differs  from  limestone  in  its  greater 
hardness  and  feebler  solubility  in  acid,  in  its  frequently  cellular 
or  cavernous  texture,  in  its  tendency  to  assume  spherical,  grape- 
shaped,  or  other  irregular  concretionary  forms  (Fig.  75),  and 
in  its  proneness  to  crumble  down  into  loose  crystals.  It  occurs 
in  beds,  not  uncommonly  associated  with  gypsum  and  rock-salt, 
and  in  such  conditions  it  may  have  been  deposited  first  as 
limestone  which,  by  the  chemical  action  of  the  magnesian  salts 
in  the  saline  water,  had  its  carbonate  of  lime  partially  replaced 
by  carbonate  of  magnesia.  It  is  also  found  in  irregular  bands 
traversing  limestone  which,  probably  by  the  influence  of  per- 


1G2  GEOLOGY 

colating  water  containing  carbonate  of  magnesia  in  solution, 
has  been  changed  into  dolomite. 

GYPSUM  is  not  only  a  mineral  (p.  140)  but  also  a  rock,  white, 
gray,  brown,  or  reddish  in  colour,  granular  to  compact,  some- 
times fibrous  or  coarsely  crystalline  in  texture.  It  consists  of 
sulphate  of  lime,  is  easily  scratched  with  the  nail,  and  is  not 
affected  by  acids,  being  thus  readily  distinguishable  from  lime- 
stone. It  is  found  in  beds  or  veins,  especially  associated  with 
layers  of  red  clay  and  rock-salt,  and  in  these  cases  has  evidently 
resulted  from  the  evaporation  of  water  containing  it  in  solution, 
such  as  that  of  the  sea.  The  lime-sulphate  being  less  soluble 


Fig.  75. —  Concretionary  forms  assumed  by  Dolomite,  Magnesian  Lime- 
stone,  Durham. 

than  the  other  constituents  is  precipitated  first.  Hence  in  a 
thick  series  of  alternations  of  beds  of  gypsum  (or  anhydrite) 
and  rock-salt,  each  layer  of  sulphate  of  lime  indicates  a  new 
supply  of  water  into  the  natural  reservoirs  where  the  evapora- 
tion took  place.  The  overlying  bed  of  salt,  usually  much  thicker 
than  the  gypsum,  points  to  the  condensation  of  the  water  into 
a  strong  brine,  from  which  the  salt  was  ultimately  precipitated. 
And  the  next  sheet  of  sulphate  of  lime  tells  how,  by  the  break- 
ing down  of  the  barrier,  renewed  supplies  of  salt  water  were 
poured  into  the  basin. 


CLASSIFICATION  OF  ROCKS  103 

BOCK-SALT  occurs  in  beds  or  layers,  from  less  than  an  inch 
to  hundreds  or  even  thousands  of  feet  in  thickness.  One  mass 
of  salt  in  Galicia  is  more  than  4600  feet  thick,  and  a  still 
thicker  mass  occurs  near  Berlin.  When  quite  pure,  rock-salt 
is  clear  and  colourless,  but  it  is  usually  more  or  less  mixed  with 
impurities,  particularly  with  red  clay,  and  in  association  with 
beds  of  gypsum,  as  above  remarked.  It  has  been  formed  in 
inland  salt  lakes  or  basins  by  the  evaporation  and  concentration 
of  the  saline  water.  It  is  being  deposited  at  the  present  time 
in  the  Dead  Sea.,  the  Great  Salt  Lake,  and  the  salt  lakes  so 
frequent  in  the  desert  regions  of  continents,  where  the  drainage 
does  not  flow  outwards  to  the  sea. 

IRONSTONE. —  Various  minerals  are  included  under  this  name 
as  large  rock-masses.  One  of  the  most  important  of  them  is 
Hcematite  (p.  133),  which  occurs  in  large  beds  and  veins,  as 
well  as  filling  up  caverns  in  limestone.  Limonite  or  bog-iron 
ore  is  formed  in  lakes  and  marshy  places  (p.  134),  and  occurs 
in  beds  among  other  sedimentary  accumulations.  Magnetite 
(p.  134)  is  found  in  beds  and  huge  wedge-shaped  masses  among 
various  crystalline  rocks,  as  in  Scandinavia,  where  it  sometimes 
forms  an  entire  mountain.  Carbonate  of  iron  (Siderite,  Sphae- 
rosiderite,  Clay-ironstone)  occurs  in  concretions  and  beds  among 
argillaceous  deposits  (Figs.  61,  65,  and  p.  140).  In  the  Coal- 
measures,  for  example,  it  is  largely  developed,  much  of  the 
iron  of  Britain  being  obtained  from  this  source.  As  many  iron- 
stones are  largely  due  to  the  influence  of  plants  and  animals, 
the  rock  is  alluded  to  again  on  p.  166. 

SILICEOUS  SINTER  —  a  white  powdery  to  compact  and  flinty 
deposit  from  the  hot  water  of  springs  in  volcanic  districts,  con- 
sisting of  84  to  91  per  cent  of  silica,  with  small  proportions  of 
alumina,  peroxide  of  iron,  lime,  magnesia,  and  alkali,  and  from 
5  to  8  per  cent  of  water.  It  accumulates  in  basin-shaped 
cavities  round  the  mouths  of  hot  springs  and  geysers,  and 
sometimes  forms  extensive  terraces  and  mounds,  as  at  the  geyser 
regions  of  Iceland,  Wyoming,  and  New  Zealand. 

VEIN-QUARTZ  —  a  massive  form  of  quartz,  which  occurs  in 
thin  veins  and  in  broad  dyke-like  reefs,  traversing  especially 
the  older  rocks. 


164  GEOLOGY 

(3)  Rocks  formed  of  the  Remains  of  Plants  or  Animals. 

In  Chapter  VIII  an  account  was  given  of  t  the  manner  in 
which  extensive  accumulations  are  now  being  formed  of  the 
remains  of  plants  and  animals.  Similar  deposits  have  con- 
stantly been  accumulated  from  an  early  period  in  the  history 
of  the  earth.  Eegarding  them  with  reference  to  their  mode 
of  origin,  we  observe  that  in  some  cases  they  have  been  piled 
up  by  the  unremitting  growth  and  decay  of  organisms  upon 
the  same  site.  In  a  thick  coral-reef,  for  example,  the  living 
corals  now  building  on  the  surface  are  the  descendants  of  those 
whose  skeletons  form  the  coral-rock  far  below  them.  In 
other  cases,  the  remains  of  the  organisms  are  broken  up  and 
carried  along  by  moving  water,  which  deposits  them  elsewhere 
as  a  sediment.  Strictly  speaking,  these  last  deposits  are  frag- 
mental,  and  might  be  classed  with  those  described  at  p.  154; 
they  pass  into  ordinary  sand,  sandstone,  clay,  or  shale.  But- 
it  will  be  more  convenient  to  class  together  all  the  rocks  which 
consist  mainly  of  organic  remains,  whether  they  have  been 
directly  built  up  by  the  organisms,  or  have  only  been  formed 
out  of  their  detrital  remains. 

LIMESTONE. —  As  carbonate  of  lime  is  so  largely  secreted  by 
animals  in  their  hard  parts  which  are  more  or  less  durable, 
it  is  naturally  the  most  common  substance  among  rocks  of 
organic  origin.  The  limestones  that  form  so  large  a  proportion 
of  the  stratified  rocks  of  the  earth's  crust  have  been,  for  the 
most  part,  formed  out  of  the  remains  of  marine  animals.  The 
following  are  some  of  the  more  important  or  interesting  varieties 
of  this  rock : —  Shell-marl.,  a  soft  white  earthy  crumbling  de- 
posit which  is  formed  chiefly  of  fresh-water  shells;  by  sub- 
sequent infiltration  it  may  be  hardened  into  a  compact  stone, 
when  it  is  known  as  fresh-water  limestone;  Calcareous  sand  — 
a  mass  of  broken-up  shells,  calcareous  algae,  and  other  calcareous 
organisms  which  are  often  cemented  by  percolating  water  into 
solid  stone ;  Coral  rock  —  a  limestone  formed  by  the  continuous 
growth  o'f  corals  and  cemented  into  a  solid  compact  and  even 
crystalline  rock  by  the  washing  of  calcareous  mud  into  its 
interstices  and  the  permeation  of  sea-water  and  rain-water 
through  it,  whereby  crystalline  calcite  is  deposited  within  it; 
Chalk  —  a  soft  and  white  rock,  which  soils  the  fingers,  formed 


CLASSIFICATION  OF  ROCKS  165 

of  a  fine  calcareous  powder  of  remains  of  foraminifera,  shells, 
etc.  (see  Ooze,  Ch.VIII)  ;  Crinoidal  limestone  —  composed  chief- 
ly of  the  calcareous  joints  of  the  marine  creatures  known  as 
crinoids,  with  foraminifera,  shells,  corals,  and  other  organisms. 
A  limestone  composed  in  great  part  of  organic  remains  may 
show  little  trace  of  its  origin  on  a  fresh  fracture  of  the  stone; 
but  a  weathered  surface  will  often  reveal  its  true  nature,  the 
fossils  being  better  able  to  withstand  the  action  of  the  atmos- 
phere than  the  surrounding  matrix  which  is  accordingly  re- 
moved, leaving  them  standing  out  in  relief  (Fig.  76). 

PEAT  —  a  yellow,  brown,  or  black  fibrous  mass  of  compressed 
and  somewhat  altered  vegetation.  It  occurs  in  boggy  places 
in  temperate  latitudes  where  it  largely  consists  of  bog-mosses 


Fig.   76. —  Weathered  surface  of  crinoidal    limestone. 


and  also  of  other  marshy  plants.  Its  upper  parts  are  loose 
and  full  of  the  roots  of  living  plants,  while  the  bottom  portions 
may  be  compact  and  black  like  clay,  and  with  little  trace  of 
vegetable  structure. 

LIGNITE  or  Brown  Coal  is  a  more  compressed  and  chemically 
changed  condition  of  vegetation.  It  varies  in  colour  from 
yellow  to  deep  brown  or  black,  and  may  be  regarded  as  an  inter- 
mediate stage  between  peat  and  coal.  -It  occurs  in  beds  inter- 
calated between  layers  of  shale,  clay,  and  sandstone. ' 

COAL  —  a  compact,  brittle,  black,  or  dark  brown  stone,  formed 
of  mineralised  vegetation,  and  found  in  beds  or  seams  usually 
resting  on  clay,  and  covered  with  sandstone,  shale,  etc.  (see 
Figs.  79  and  140).  There  are  many  varieties  of  coal,  differing 


166  GEOLOGY 

from  each  other  in  the  relative  proportions  of  their  constituents. 
Coking-coal,  such  as  is  ordinarily  used  in  England,  contains 
from  75  to  80  per  cent  of  carbon,  5  or  6  per  cent  of  hydrogen, 
and  10  or  12  per  cent  of  oxygen,  with  some  sulphur  and  other 
impurities.  Anthracite,  the  most  thoroughly  mineralised  con- 
dition of  vegetation,  is  a  hard,  brittle,  lustrous  substance,  from 
which  the  hydrogen  and  oxygen  have  been  in  great  measure 
driven  away,  leaving  90  per  cent  or  more  of  carbon. 

IRONSTONE. —  Reference  has  been  made  before  to  ironstone 
iprecipitated  from  chemical  solution.  This  precipitation  is  often 
caused  through  the  medium  of  decomposing  organic  matter. 
Organic  acids,  produced  by  the  decay  of  plants  in  marshy  places 
and  shallow  lakes,  attack  the  salts  of  iron  contained  in  the 
rocks1  or  detritus  of  the  bottom,  and  remove  the  iron  in  solution. 
On  exposure,  the  iron  oxidises  and  is  thrown  down  as  a  yellow 
or  brown  precipitate  of  limonite  or  bog-iron  ore  (p.  134), 
which  is  found  in  layers  and  concretions.  Clay-ironstone,  com- 
posed of  a  mixture  of  carbonate  of  iron,  with  clay  and  carbona- 
ceous matter,  occurs  abundantly  both  as  nodules  and  in  layers, 
with  remains  of  plants,  shells,  fishes,  etc.,  in  the  Coal-measures 
(Figs.  61,  65,  and  bed  13  in  Fig.  80),  and  has,  no  doubt,  been 
also  formed  through  the  agency  of  organic  acids  which,  passing 
into  carbonic  acid,  have  given  rise  to  the  solution  and  subse- 
quent deposit  of  the  iron  as  carbonate  mingled  with  mud  and 
with  entombed  plants  and  animals. 

FLINT. —  Some  siliceous  deposits,  due  to  organic  agency,  have 
been  already  referred  to  in  Chap.  VIII.  Besides  these,  mention 
may  be  made  of  Flint,  which  occurs  as  dark  lumps  and  irregular 
nodular  sheets  in  chalk  and  other  limestones,  frequently  enclos- 
ing urchins,  shells,  and  other  organisms,  which  are  sometimes 
converted  into  flint.  Its  mode  or  origin  is  not  yet  thoroughly 
understood,  but  there  is  reason  to  regard  it  as  due  to  the 
abstraction  of  silica  from  sea-water,  either  directly,  by  such 
animals  as  sponges,  or  indirectly,  by  the  decomposition  of  ani- 
mal remains.  Chert  is  a  more  impure  siliceous  aggregate  found 
under  similar  conditions,  especially  among  the  older  limestones. 

GUANO  —  a  brown,  light,  powdery  deposit,  formed  of  the 
droppings  of  sea-birds  in  rainless  tracts  of  the  west  coasts  of 
South  America  and  Africa.  Containing  much  phosphate  of 


CLASSIFICATION  OF  ROCKS  1G7 

lime  as  well  as  ammoniacal  salts,  it  has  great  commercial  value 
as  an  important  manure. 

BONE-BEDS  —  deposits  composed  of  fragmentary  or  entire 
bones  of  fish,  reptiles,  or  higher  animals,  as  in  the  well-known 
bone-bed  of  the  Khaetic  series  (Ch.  XXII).  The  floors  of  some 
caverns  are  covered  with  stalagmite,  so  full  of  pieces  of  the 
bones  of  cave-bears,  hyaenas,  and  other  extinct  and  living  species, 
as  to  be  called  Bone-breccia.  Layers  of  stone,  full  of  the  copro- 
lites  (fossil  excrement)  or  of  the  rolled  bones  of  various  verte- 
brate animals  have,  in  recent  years,  been  largely  worked  as 
sources  of  phosphate  of  lime  for  the  manufacture  of  artificial 
manures. 

II.  ERUPTIVE  EOCKS. 

Under  this  division  are  grouped  all  the  massive  rocks  which 
have  been  erupted  from  underneath  into  the  crust  or  to  the 
surface  of  the  earth.  They  are  composed  chiefly  of  silicates 
of  alumina,  magnesia,  lime,  potash,  and  soda,  with  different 
proportions  of  free  silica,  magnetic  or  other  oxide  of  iron,  and 
phosphate  of  lime.  The  principal  silicate  is  generally  some 
felspar,  the  number  of  eruptive  rocks  without  felspar  being 
comparatively  small.  The  felspar  is,  in  different  rocks,  con- 
joined with  mica,  hornblende,  augite,  magnetite,  or  other 
minerals. 

No  perfectly  satisfactory  classification  of  the  eruptive  rocks 
has  yet  been  devised;  they  have  been  grouped  according  to 
their  presumed  mode  of  origin,  some  being  classed  as  plutonic 
or  Jiypogene.,  from  their  supposed  origin,  deep  within  the  earth's 
crust,  others  as  volcanic,  from  having  been  ejected  by  volcanoes. 
They  have  likewise  been  arranged  according  to  their  chemical 
composition,  and  also  with  reference  to  their  internal  structure. 
In  the  following  enumeration  of  some  of  the  more  abundant 
and  important  varieties,  it  may  be  enough  to  adopt  an  arrange- 
ment in  three  sections,  according  to  the  nature  of  the  pre- 
dominant silicate:  viz.  (1)  Orthoclase  rocks;  (2)  Plagioclase 
rocks;  and  (3)  Olivine  and  Serpentine  rocks.  It  has  already 
been  pointed  out  that  the  original  condition  of  many  lavas  and 
other  eruptive  rocks  has  been  that  of  molten  glass,  their  present 
stony  structure  being  due  to  the  more  or  less  complete  devitrifi- 


108  GEOLOGY 

cation  and  disappearance  of  the  glass  by  the  development  of 
crystals  and  crystallites  out  of  it  during  the  process  of  cooling 
and  consolidation  (p.  149).  Though  there  is  no  evidence  that 
all  crystalline  eruptive  rocks  have  once  been  in  the  state  of 
molten  glass,  it  may  be  useful  to  begin  with  the  vitreous  va- 
rieties, which  we  know  to  represent  the  earliest  forms  of  many 
that  are  now  quite  crystalline. 

(1)   Orthoclase  Rocks. 

In  this  section  the  prevalent  silicate  is  Orthoclase,  either  in 
its  common  dull,  white,  or  pink  form,  or  in  the  glassy  condition 
(sanidine).  In  many  of  the  rocks,  free  quartz  occurs  either  in 
irregular  crystalline  blebs  or  in  definite  crystals,  which  fre- 
quently take  the  form  of  double  pyramids.  Among  other 
minerals,  horn-blende,  white  and  black  mica,  and  apatite  are 
of  common  occurrence.  The  rocks  of  this  division  are  the 
most  acid  of  the  eruptive  series  —  that  is,  they  contain  the 
largest  proportion  of  silica  or  silicic  acid,  sometimes  more  than 
75  per  cent.  Some  of  them  (granite)  are  only  found  as  masses 
that  have  consolidated  deep  beneath  the  surface;  others 
(trachyte,  rhyolite,  obsidian)  are  abundant  as  superficial  vol- 
canic products. 

OBSIDIAN  —  a  black,  brown,  or  greenish  (sometimes  yellow, 
blue,  or  red)  glass,  breaking  with  a  shell-like  or  conclwidal 
fracture  and  into  sharp  splinters,  which  are  translucent  at  the 
edges.  .Examined  in  a  thin  section  under  the  microscope,  the 
rock  is  found  to ,  owe  its  usual  blackness  to  the  presence  of 
minute  opaque  crystallites  (Fig.  69)  which  are  crowded  through 
it,  not  infrequently  drawn  out  into  streaky  lines  and  curving 
round  any  larger  crystal  that  may  be  embedded  in  the  mass 
(Fig.  71  B).  These  arrangements,  called  flow-structure  (p. 
152,  have  evidently  been  caused  by  the  movement  of  the  rock 
while  still  in  a  fused  state,  the  crystallites  and  other  objects 
being  borne  onward  by  the  currents  of  molten  glass.  In  some 
obsidians,  little  spherulites  of  a  dull  gray  enamel-like  substance 
have  made  their  appearance  as  stages  in  the  devitrification  of 
the  rock  (Fig.  71)  ;  but  the  mass  has  consolidated  before  the 
stony  condition  could  be  completed.  In  other  instances,  the 
whole  rock  has  passed  into  a  stony  enamel-like  mass  with 


CLASSIFICATION  OF  ROCKS  169 

perlitic  structure  (pearlstone,  p.  150).  Where  a  still  molten 
obsidian  has  been  frothed  up  by  the  expansion  of  steam  or 
gas  through  it,  so  as  to  become  a  spongy  cellular  substance* 
which  will  float  on  water,  it  is  called  pumice.  Obsidian  occurs 
in  many  volcanic  regions,  sometimes  as  streams  of  lava  which 
have  been  poured  forth  at  the  surface,  sometimes  in  dykes  and 
veins,  and  often  in  fragments  ejected  with  the  other  detritus 
that  now  forms  tuff. 

TRACHYTE  —  a  compact  porphyritic  rock,  consisting  mainly 
of  orthoclase  (sanidine),  with  some  plagioclase  and  usually  with 
some  hornblende,  or  with  augite,  mica,  magnetite,  or  other 
minerals ;  having  a  peculiar  matrix  which,  under  the  microscope, 
is  found  to  consist  mainly  of  minute  felspar-crystallites.  Large 
crystals  of  orthoclase  (sanidine)  are  frequent,  and  also  scales 
of  dark  mica.  This  rock  is  found  abundantly  among  some  of 
the  younger  volcanic  regions  of  the  world,  where  it  occurs  in 
lava  streams  and  also  in  intrusive  sheets  and  dykes.  QUARTZ- 
TRACHYTE  (Liparite,  Rhyolite)  is  a  rock  composed  of  a  com- 
pact, often  rough  and  somewhat  porous  base,  through  which 
are  scattered  crystals  of  felspar  and  blebs  of  quartz,  often  also 
with  hornblende  and  mica. 

FELSITE  —  an  exceedingly  close-grained  rock,  composed  of  an 
intimate  mixture  of  quartz  and  orthoclase.  The  felspar  often 
occurs  as  large  disseminated  crystals,  giving  the  porphyritic 
structure.  Where  the  quartz  appears  as  distinct  blebs  or  crystals 
(sometimes  double  pyramids)  the  rock  becomes  QUARTZ- 
PORPHYRY.  The  felsites  and  quartz-porphyries  play  an  impor- 
tant part  among  the  eruptive  rocks  of  older  geological,  time, 
occurring  both  in  the  form  of  lavas  erupted  to  the  surface  and 
of  intrusive  masses  that  have  consolidated  below  ground. 
Many  of  them  can  be  proved  to  have  been  originally  in  the 
condition  of  molten  glass  which  has  been  devitrified.  Eocks 
which  show  the  characteristic  closeness  of  grain  characteristic 
of  the  felsites  are  said  to  be  felsitic  or  to  have  a  felsitic  ground 
mass. 

SYENITE  —  a  thoroughly  crystalline  rock,  consisting  essen- 
tially of  orthoclase  and  hornblende,  and  distinguished  from 
granite  chiefly  by  the  absence  or  small  amount  of  quartz.  It 
occurs  in  bosses  and  veins  which  have  been  erupted  into  older 
rocks. 


170  GEOLOGY 

GRANITE  —  a  thoroughly  crystalline  (holo-crystalline)  com- 
pound of  felspar,  quartz,  and  mica,  the  individual  minerals 
being  large  enough  to  be  distinctly  recognised  by  the  naked  eye. 
Sometimes  large  crystals  of  felspar  are  porphyritically  scattered 
through  the  rock.  Granite  occurs  in  large  eruptive  masses 
which  have  been  intruded  into  many  different  kinds  of  rocks, 
also  in  smaller  bosses  and  veins.  Round  the  outside  of  a  mass 
of  granite  there  frequently  diverge  from  it  dykes  and  veins 
which,  where  they  are  of  great  width,  may  show  the  usual 
granitic  structure;  but  which,  when  of  small  dimensions,  are 
apt  to  appear  as  felsite  or  quartz-porphyry.  There  can  be  no 
doubt  that  such  fine-grained  veins  are  actually  portions  of  the 
same  mass  of  rock  as  the  granite,  so  that  granite  and  felsite 


Fig.  77. —  Group  of  crystals  of  felspar,  quartz,  and  mica,  from  a  cavity 
in  the  Mourne  Mountain  granite. 

or  quartz-porphyry  are  only  different  conditions  of  the  same 
substance,  the  differences  being  probably  due  to  variations  in 
the  circumstances  under  which  the  cooling  and  consolidation 
took  place.  In  the  crystalline-granular  structure  so  distinctive 
of  granite  (granitic  or  granitoid,,  p.  149)  the  constituent  min- 
erals have  not  had  room  to  assume  perfect  crystallised  shapes, 
but  occasionally  they  have  been  able  to  shoot  out  in  perfect 
crystals  where  cavities  occur.  Fig.  77,  for  example,  shows  a 
group  of  the  ordinary  crystals  of  this  rock  which  have  crystal- 
lised in  a  cavity  of  the  granite  of  the  Mourne  Mountains, 
Ireland.  It  is  in  such  cavities  also  that  the  rarer  minerals  of 
this  rock,  such  as  topaz  and  beryl,  may  be  looked  for. 


CLASSIFICATION  OF  ROCKS  171 

(2)     Plagiodase  Rocks. 

In  this  section  the  felspar  is  some  variety  of  plagioclase,  and 
the  other  most  frequent  silicate  is  either  augite  or  hornblende. 
Though  free  quartz  occurs  in  some  of  the  rocks,  they  contain 
generally  so  much  less  silica  than  the  orthoclase  rocks  that 
instead  of  being  acid  they  are  commonly  basic  compounds.  A 
range  of  texture  can  be  observed  in  them  similar  to  that  char- 
acteristic of  the  orthoclase  series,  from  a  true  glass  up  to  a 
thoroughly  crystalline  granitoid  rock.  Some  of  them,  more 
especially  the  coarsely  crystalline  varieties,  are  probably  of 
deep-seated  origin;  others  (and  these  include  the  great  ma- 
jority) are  truly  volcanic  ejections  which  have  risen  in  volcanic 
pipes  and  fissures,  and  have  been  poured  forth  at  the  surface 
as  actual  lava-streams. 

BASALT-ROCKS  —  a  group  of  rocks  consisting  of  plagioclase, 
augite,  olivine,  and  magnetite  or  titaniferous  iron,  to  which 
apatite  and  other  minerals  may  be  added.  These  rocks  range 
in  texture  from  a  black  glass  up  to  a  coarsely  crystalline  mass 
wherein  the  component  minerals  are  distinctly  visible  to  the 
naked  eye.  Different  names  are  employed  to  distinguish  these 
varieties.  Basalt-glass  (Tachylyte,  Hyalomelan)  is  a  general 
epithet  to  denote  the  vitreous  varieties.  These  are  particularly 
to  be  observed  along  the  edges  of  dykes  and  other  intrusive 
masses,  where  they  represent  the  outer  surface  of  the  basalt  that 
was  suddenly  chilled  and  consolidated  by  coming  in  contact  with 
the  cold  walls  of  the  vent  or  fissure  into  which  it  was  injected, 
and  where  they  no  doubt  show  what  was  the  original  state  of 
the  whole  basalt  before  devitrification  converted  the  rock  into 
its  present  crystalline  structure  (see  Chap.  IX).  Basalt  — 
a  black,  compact,  heavy,  homogeneous  rock,  breaking  with  a 
conchoidal  fracture,  showing  sometimes  large  porphyritic 
crystals  of  plagioclase,  olivine,  or  augite,  but  too  fine-grained 
for  the  component  minerals  of  the  base  to  be  determined  except 
with  the  microscope.  The  coarser  varieties,  where  the  minerals 
can  be  recognised  with  the  naked  eye,  are  known  as  Dolerite. 
The  basalt-rocks  are  pre-eminently  volcanic  lavas,  occurring 
both  as  intrusive  masses  that  consolidated  underground,  and  as 
sheets  that  were  poured  out  in  successive  streams  at  the  surface. 
The  black,  compact  kinds  (true  basalt)  are  particularly  prone 


172 


GEOLOGY 


CLASSIFICATION  OF  ROCKS  173 

to  assume  columnar  forms  (Fig.  78),  whence  columnar  rocks 
are  sometimes  spoken  of  as  basaltic.  In  some  varieties  of  basalt 
the  mineral  leucite  takes  the  part  of  the  plagioclase;  and  in 
others  this  is  done  by  another  mineral,  nepheline. 

DIABASE  —  a  name  given  to  some  ancient  basalt-rocks  in 
which,  owing  to  alteration  of  their  augite  or  olivine,  a  greenish 
chloritic  discoloration  has  often  taken  place.  The  lavas  of 
early  geological  time  are  to  a  large  extent  diabase. 

ANDESITE  is  closely  allied  to  basalt;  but  contains  no  olivine. 
It  sometimes  includes  free  quartz,  and  hornblende  may  be. sub- 
stituted in  it  for  augite.  Hornblende-andesite  and  Augite-ande- 
site  are  lavas  which  have  been  extensively  erupted  in  later 
geological  time. 

DIORITE  —  a  crystalline  aggregate  of  plagioclase  and  horn- 
blende, usually  with  magnetite  and  apatite,  sometimes  with 
augite  and  mica.  The  hornblende  is  black  or  dark  green  and 
often  more  or  less  decomposed,  giving  rise  to  a  greenish  chloritic 
discoloration  of  the  felspar.  From  its  prevalent  green  colour, 
the  rock  was  formerly  known  as  "greenstone."  It  occurs  in 
intrusive  masses,  and  seems  generally  if  not  always  to  have 
consolidated  below  ground  instead  of  being  poured  out  at  the 
surface. 

GABBRO,  DIALLAGE-ROCK  —  a  thoroughly  crystalline  granitoid 
aggregate  of  plagioclase  and  the  variety  of  augite  known  as 
diallage,  which  appears  in  distinct  brown  or  greenish  crystals, 
with  a  peculiar  metalloidal  or  pearly  lustre;  it  is  found  in 
bosses  associated  with  granite,  gneiss,  etc.,  and  also  sometimes 
with  volcanic  rocks  in  centres  of  eruption. 

(3)   Olivine  and  Serpentine  Rocks. 

In  this  group  may  be  included  a  comparatively  small  number 
of  rocks  which  consist  principally  of  olivine,  and  which  by 
gradual  alteration  pass  into  serpentine  (Fig.  58).  OLIVINE- 
ROCKS  (Peridotites)  are  liable  to  remarkably  rapid  changes 
of  texture  and  composition.  In  some  places  they  are  mainly 
made  up  of  olivine,  augite,  or  hornblende,  magnetite,  and  brown 
mica,  but  some  of  these  minerals  may  disappear  and  some 
felspar  may  take  their  place.  They  are  intrusive  masses  which 
appear  to  have  been  generally  injected  into  the  crust  in  con- 


174  GEOLOGY 

nection  with  volcanic  eruptions,  rather  than  to  have  been  poured 
out  at  the  surface  in  true  lava-streams. 

SERPENTINE  —  a  compact,  dull,  or  faintly  glimmering  rock, 
with  a  general  dark  dirty  green  colour,  variously  mottled, 
greasy  to  the  touch,  easily  scratched,  and  giving  a  white  powder 
which  does  not  effervesce  with  acids.  It  is  a  massive  form  of 
the  mineral  serpentine  described  on  p.  139,  frequently  con- 
taining disseminated  crystals  of  the  minerals  bronzite,  enstatite, 
and  chromic  iron,  and  veins  of  a  delicately  fibrous  silky  variety 
of  serpentine  known  as  chrysotile.  Many  serpentines  were 
originally  olivine-rocks  which,  by  hydration  and  alteration  of 
their  magnesian  silicates,  have  assumed  their  present  characters. 
Serpentine  occurs  in  bosses,  dykes,  and  veins,  which  were  evi- 
dently of  eruptive  origin  and  were  at  first  probably  olivine- 
rocks;  it  is  also  found  in  thick  beds  associated  with  limestones 
and  crystalline  schists,  where  it  may  be  a  metamorphosed  sedi- 
mentary rock. 

III.  THE  SCHISTS  AND  THEIR  ACCOMPANIMENTS. 

This  section  includes  a  remarkable  series  of  rocks  of  which 
the  leading  feature  is  the  possession  of  a  schistose  or  foliated 
character  (Fig.  72 )\  They  are,  in  their  more  typical  varieties, 
distinctly  crystalline.  Some  of  them  shade  off  into  ordinary 
fragmental  rocks,  such  as  shale  and  sandstone;  others  agree 
in  chemical  and  mineral  composition  with  some  of  the  eruptive 
rocks  already  enumerated,  into  which  they  may  often  be  traced 
by  imperceptible  gradations. 

In  the  schists,  therefore,  we  see  an  assemblage  of  rocks  which, 
though  possessing  distinct  characters  of  their  own,  may  yet  be 
observed  to  shade  off  into  fragmental  rocks  on  the  one  side, 
and  into  eruptive  rocks  on  the  other.  In  Chapter  XIII  some 
further  account  of  them  will  be  given,  with  special  reference 
to  their  probable  origin,  and  to  the  grounds  on  which  they  have 
been  regarded  as  metamorphic  or  altered  rocks.  For  the  pres- 
ent, in  taking  notice  of  their  composition  and  structure,  it  will 
be  enough  to  state  that  in  many  cases  they  can  be  shown  to 
be  more  or  less  altered  and  crystalline  transformations  of  what 
were  originally  sedimentary  rocks;  and  that  in  other  instances 
they  represent  original  crystalline  eruptive  masses,  which  have 


CLASSIFICATION  OF  ROCKS  175 

been  subjected  to  such  enormous  pressure  and  shearing,  that  a 
foliated  structure  and  recrystallisation  of  minerals  have  been 
superinduced  in  them.  The  essential  feature  which  unites 
masses  of  such  different  origin  is  the  possession  of  that  common 
schistose  structure  which  they  have  derived  from  having  all 
been  alike  subjected  to  the  same  kind  of  intense  terrestrial 
movements. 

CLAY-SLATE  —  a  hard  fissile  clay-rock,  through  which  minute 
scales  of  mica  and  crystals  or  crystallites  of  other  minerals  have 
been  developed;  generally  bluish-gray  to  purple  or  green,  and 
splitting  into  thin  parallel  leaves.  As  this  rock  often  contains 
remains  of  marine  animals  and  plants,  and  is  interstratified 
with  bands  of  sandstone,  grit,  conglomerate,  and  limestone,  it 
was  undoubtedly  at  first  in  the  condition  of  soft  mud  on  the 
sea-bottom.  Sometimes  the  organic  remains  in  it  are  so  curi- 
ously elongated  or  distorted  in  one  general  direction  as  to  show 
that  the  rock  has  been  drawn  out  by  intense  pressure  and 
shearing  (Figs.  98,  103,.  104).  The  planes  along  which  clay- 
slate  splits  are  generally  independent  of  the  original  surfaces 
of  deposit,  sometimes  cross  these  at  a  right  angle,  and  have 
been  superinduced  by  mechanical  movements  (Cleavage),  as 
explained  in  Chapter  XIII.  Different  varieties  of  clay-slate 
have  received  special  names.  Roofing  slate  is  the  fine  compact 
durable  kind,  employed  for  roofing  purposes  and  also  for  the 
manufacture  of  cisterns,  chimneypieces,  writing-slates;  Alum' 
slate  —  dark,  carbonaceous,  and  pyritous,  the  iron-disulphide 
oxidising  into  sulphuric  acid,  and  giving  rise  to  an  efflorescence 
of  alum;  Wliet-slate  Jionestone  —  exceedingly  hard,  fine-grained, 
and  suitable  for  making  hones;  sometimes  owing  its  hardness 
to. the  presence  of  microscopic  crystals  of  garnet;  Chiastolite- 
slate  —  containing  disseminated  crystals  of  chiastolite,  and 
found  especially  around  eruptive  bosses  of  granite.  By  increase 
of  its  mica-flakes  a  clay-slate  passes  into  a  Phyllite,  -which  has  a 
more  silvery  sheen,  and  represents  a  farther  stage  of  meta- 
morphism.  Phyllite,  by  increase  of  the  mica,  becomes  Mica- 
slate,  so  that  a  transition  may  be  traced  from  sedimentary  fossil- 
iferous  rocks  through  clay-slate  and  phyllite  into  thoroughly 
crystalline  schist.  Clay-slate  occurs  extensively  among  the 
older  geological  formations  in  all  parts  of  the  world. 


176  GEOLOGY 

AMPHIBOLITES  —  rocks  composed  mainly  of  hornblende,  but 
with  quartz,  orthoclase,  and  other  minerals  in  minor  propor- 
tions; sometimes  they  are  massive  and  granular  (Hornblende- 
rock),  and  in  this  condition  doubtless  represent  eruptive  rocks. 
Gradations  can  be  followed  from  such  rocks  (originally  diorite, 
diabase,  etc.)  into  perfect  schist  (Hornblende-schist),  so  that 
the  development  of  the  schistose  structure  can  be  traced  from 
rocks  that  were  at  first  as  structureless  as  any  amorphous  erup- 
tive mass  can  be.  Amphibolites  occur  among  the  crystalline 
schists  in  most  parts  of  the  world  as  occasional  bands  or  bosses, 
which  probably  mark  zones  of  basic  igneous  rock,  either  in- 
truded into  the  accompanying  masses,  or  contemporaneously 
erupted  with  them. 

CHLORIDE-SCHIST  —  a  scaly,  schistose  aggregate  of  greenish 
chlorite  with  quartz,  and  often  with  felspar,  mica,  and  octahedra 
of  magnetite  (Fig.  54) ;  it  occurs  in  beds  associated  with  gneiss 
and  other  schists.  Some  chloritic  schists  may  represent  old 
lavas  or  other  erupted  rocks  which  have  been  crushed  down 
and  become  schistose;  others,  especially  where  they  contain 
pebbles  of  quartz,  etc.,  and  are  banded  with  quartzites  and 
schistose  conglomerates,  not  improbably  mark  where  fine  vol- 
canic ashes  fell  over  a  sea-bottom,  and  were  then  mingled  and 
interstratified  with  the  ordinary  sediment  that  happened  to  be 
accumulating  at  the  time. 

MICA-SCHIST  (MICA-SLATE)  —  a  schistose  aggregate  of  quartz 
and  mica,  the  two  minerals  being  arranged  in  irregular  but 
nearly  parallel  wavy  folia.  The  rock  splits  along  the  laminaB 
of  mica,  so  that  its  flat  surfaces  have  a  bright  silvery  sheen, 
and  the  quartz  is  not  well  seen  except  on  the  cross  fracture, 
where  only  the  thin  edges  of  the  mica-plates  present  themselves. 
Mica-schist  is  often  remarkably  crumpled  or  puckered  —  a 
structure  bearing  witness  to  the  intense  compression  it  has 
undergone  (Fig.  114).  It  abounds  in  most  regions  where 
schists  are  extensively  developed  (Chapter  XVI).  Some  mica- 
schists  contain  fossil  shells  and  corals  (Bergen),  and  must  thus 
represent  what  were  originally  sedimentary  deposits ;  others  may 
be  highly  deformed  eruptive  rocks. 

GNEISS  —  a   schistose   aggregate   of   orthoclase,   quartz,   and 


CLASSIFICATION  OF  ROCKS  177 

mica,  varying  in  texture  from  a  fine-grained  rock  up  to.  a 
coarse  crystalline  mass  which,  in  hand  specimens,  may  not  be 
distinguishable  from  granite.  There  is  no  difference  indeed 
as  regards  composition  between  gneiss  and  granite;  gneiss  may 
be  called  a  foliated  granite.  There  is  good  reason  to  believe 
that  some,  if  not  all,  true  gneisses  have  been  made  out  of  granite 
or  allied  rocks  by  the  process  of  shearing  above  referred  to. 
Gneiss  occurs  abundantly  among  the  oldest  known  rocks  of  the 
earth's  crust,  and  may  be  found  in  most  large  regions  of  crystal- 
line schists  (Chapter  XVI). 

A  few  rocks  which  are  found  associated  with  the  schists,  or 
with  evidence  of  metamorphism,  may  be  noticed  here  —  marble, 
quartzite,  and  schistose  conglomerate. 

MARBLE  —  a  crystalline  granular  aggregate  of  calcite,  white 
when  pure,  and  having  the  texture  of  loaf-sugar,  but  passing, 
into  various  colours  according  to  the  nature  of  the  impurities. 
It  occurs  in  beds  among  the  schists,  and  is  no  doubt  a  lime- 
stone, formed  either  by  chemical  precipitation  or  by  organic 
agency,  which  has  been  metamorphosed  by  heat  and  pressure 
into  its  present  thoroughly  crystalline  character.  Some  of  the 
fossiliferous  limestones  through  which  the  Christiania  granite 
rises  have  been  changed  into  crystalline  marble,  but  their 
original  corals  and  shells  have  not  been  wholly  effaced  (see 
Chapter  XIV). 

QUARTZITE  —  a  hard,  compact,  granular  rock,  composed  of 
adherent  quartz-grains,  and  breaking  with  a  characteristic 
lustrous  fracture.  It  occurs  in  beds  and  thick  masses,  not  in- 
frequently associated  with  slates,  mica-schists,  and  limestones; 
it  sometimes  contains  organic  remains;  and  is  evidently  -an 
indurated  siliceous  sand. 

SCHISTOSE  GRIT  AND  CONGLOMERATE. —  Interstratified  with 
clay-slates  and  mica-schists  there  are  sometimes  found  beds  of 
grit  and  conglomerate,  the  grains  and  pebbles  of  which  consist 
of  quartz  or  other  durable  material,  imbedded  in  slate  or  schist. 
The  original  fragmental  character  of  such  rocks  admits  of  no 
doubt;  they  were  obviously  at  one  time  sheets  of  fine  and  coarse 
gravel  mixed  with  sandy  mud;  and  their  presence  among 
schistose  rocks  furnishes  additional  corroborative  evidence  of 


178  GEOLOGY 

the  original  sedimentary  character  of  some  of  these  rocks. 
The  clay  or  mud  which  formed  the  matrix  has  been  metamor- 
phosed into  a  more  or  less  thoroughly  crystalline  micaceous 
substance,  while  in  many  cases  the  pebbles  have  been  flattened 
and  pulled  out  of  shape.  Hence  these  rocks  afford  important 
evidence  as  to  the  nature  of  the  processes  whereby  the  schists 
have  been  produced. 


SEDIMENTARY  ROCKS  179 

PAET  III. 

THE  STRUCTURE  OF  THE  CRUST  OF  THE  EARTH. 
CHAPTER  XII. 

SEDIMENTARY  ROCKS. 

HAVING   in   the  two   foregoing   chapters   considered   the 
more   important   elementary   substances    of   which   the 
earth's   crust  is   composed   and   their   combinations   in 
minerals  and  rocks,  we  have  to  inquire  how  these  minerals  and 
rocks  have  been  put  together  so  as  to  build  up  the  crust.     A  very 
little  examination  will  suffice  to  show  us  that  the  upper  or  outer 
parts  of  the  solid  globe  consist  chiefly  of  sedimentary  rocks.     All 
over  the  plains  and  low  grounds  of  the  earth's  surface,  which 
cover  so  large  a  proportion  of  the  whole  area  of  the  land,  some 
kind  of  sediment  underlies  the  soil  —  clay,  sand,  gravel,  lime- 
stone.    It  is  for  the  most  part  only  in  hilly  or  mountainous  re- 
gions that  anything  has  been  pushed  up  from  below  so  as  to  in- 
dicate the  nature  of  the  materials  underneath.     But  everywhere 
we  encounter  proofs  that  the  sedimentary  rocks  do  not  remain  as 
they  were  deposited.     In  the  first  place,  most  of  them  were  laid 
down  on  the  sea-floor  and  they  have  been  upraised  into  land.     In 
the  next  place,  not  only  have  they  been  upheaved,  they  have  not 
infrequently  been  bent,  broken,   and  crushed  until  sometimes 
their  original  condition  can  no  longer  be  determined.    Moreover 
they  have  been  invaded  by  masses  of  lava  and  other  eruptive 
rocks,  which  have  been  thrust  in  among  them  and  have  often 
burst  through  them  to  form  volcanoes  at  the  surface.   We  must 
now  endeavour  to  form  as  clear  a  conception  as  possible  of  what, 
after  all  these  changes,  the  present  structure  of  the  crust  actual- 
ly is.    In  this  chapter,  therefore,  we  may  examine  some  of  the 
leading  characters  of  sedimentary  rocks  in  the  architecture  of 
the  crust,  more  particularly  those  which  have  been  determined 
by  the  conditions  under  which  the  rocks  were  formed.     In  the 
next  chapter  we  shall  consider  some  of  the  more  important  char- 
acters which  have  been  superinduced  upon  the  rocks  since  their 
formation. 


180 


GEOLOGY 


Fig.  79. —  Section  of  strat- 
ified rocks. 


STRATIFICATION. —  It  has  been  shown  (p.  154)  that  one  of  the 
most  distinctive  features  in  sedimentary  rocks  is  that  they  are 

stratified  —  that  is,  are  arranged  in 
layers  one  above  another.  As  those 
at  the  bottom  must  have  been  depos- 
ited before  those  at  the  top,  a  suc- 
cession of  layers  of  stratified  rocks 
forms  a  record  of  deposition,  in 
which  the  early  stages  are  chronicled 
by  the  lower,  and  the  later  stages  by 
the  upper  layers.  An  illustration  of 
this  kind  of  record  has  already  been 
given  in  the  introductory  chapter. 
As  a  further  example,  the  accom- 
panying section  (Fig.  79)  may  be 
taken.  At  the  bottom  lies  a  bed  (a) 
of  dark  shale  or  clay  with  fragments  of  crinoids,  corals,  shells, 
and  other  marine  organisms.  Such  a  bed  unmistakably  points  to 
a  former  muddy  sea-floor,  on  which  the  creatures  lived  whose  re- 
mains have  been  preserved  in  the  hardened  mud  or  shale.  The 
next  bed  (6)  is  one  of  limestone  full  of  similar  organic  remains; 
it  shows  that  the  supply  of  mud,  which  had  previously  made  the 
water  turbid  and  had  been  slowly  gathering  in  successive  layers  on 
the  bottom,  now  ceased.  The  water  became  clear  and  much  better 
fitted  for  the  life  of  the  crinoids,  corals,  and  shells.  These  creat- 
ures accordingly  flourished  abundantly,  living  and  dying  on  the 
spot  generation  after  generation,  until  their  accumulated  remains 
had  built  up  a  solid  sheet  of  limestone  several  feet  thick.  But  once 
more  muddy  currents  spread  over  the  place,  and  from  the  cloud 
of  suspended  mud  there  slowly  settled  down  the  layer  of  blue 
clay  (c)  which  overlies  the  limestone.  As  hardly  any  remains 
of  organisms  are  to  be  seen  in  it,  we  may  infer  that  the  inroad 
of  mud  killed  them  off.  Next,  owing  to  some  new  shifting  of 
the  currents,  a  quantity  of  sand  was  brought  in  and  spread  out 
over  the  mud,  forming  the  sandstone  beds  (d).  The  sea  in 
which  these  various  strata  were  deposited  was  probably  shallow; 
or  its  floor  may  have  been  gradually  rising.  At  all  events,  the 
last  layers  of  sand  could  have  been  only  slightly  below  the  sur- 
face of  the  water,  for  they  are  immediately  covered  by  a  hardened 
silt  or  fire-clay  (e)  which,  from  the  abundant  roots  and  rootlets 


SEDIMENTARY  ROCKS 


181 


\      I     I 


'       '   1      1 


that  run  through  it  in  all  directions,  was  clearly  once  a  soil 
whereon  plants  grew.  It  was  probably  part 
of  a  mud-flat,  on  which  vegetation  spread  sea- 
ward from  the  land  where  the  water  shallowed, 
as  happens  at  the  present  day  among  the  trop- 
ical mangrove  swamps.  The  various  plants 
that  grew  on  this  soil  have  formed  the  coal- 
seam  (/),  no  doubt  representing  the  growth  of 
a  long  period  of  time.  But  the  existence  of 
the  coal-jungle  came  to  an  end  probably  by  a 
sinking  of  the  ground  beneath  the  water. 
Mud,  once  more  carried  hither  from  the  neigh- 
bouring land,  settled  down  upon  the  submerged 
vegetation  and  formed  the  clay  (g).  But 
that  land  plants  still  abounded  in  the  imme- 
diate neighbourhood,  is  shown  by  their  nu- 
merous remains  in  this  clay.  We  notice  too 
that  the  salts  of  iron  dissolved  in  the  water 
were  eliminated  by  the  decaying  plants  and 
animals  and  were  precipitated  in  the  form  of 
carbonate,  so  as  to  form  concretions  round 
occasional  dead  shells,  fishes,  fern-fronds,  and 
seed-cones.  What  were  the  immediately  suc- 
ceeding events  in  this  ancient  history  we  can-  pig.  80<— Section 
not  tell;  the  layer  next  in  order  is  a  coarse  ti^oThe'ds6™*" 
conglomerate  (h),  originally  gravel,  which  15.  shale.  14.  Seam 
must  have  been  swept  along  by  a  swift  cur- 
rent that  tore  away  the  upper  part  of  the 
clay-beds  (g)  and  any  strata  which  may  once 
have  overlain  them. 

The  whole  stratified  part  of  the  earth's 
crust  is  composed  of  materials  which  in  this 
way  may  be  made  to  tell  their  story.  In  forc- 
ing them  to  yield  up  their  records  of  the 
ancient  changes  of  which  they  are  memorials, 
scope  is  afforded  for  the  most  accurate  and 
laborious  investigation  and  for  the  closest  reas- 
oning from  the  facts  collected.  At  the  same 
time,  it  is  obvious  that  the  pursuit  is  one 
which  constantly  exercises  the  imagination,  and  that,  in- 


of  sandstone.  13. 
Shale  with  sep- 
tarian  nodules. 
12.  Sandstone. 
11.  Mudstone.  10. 
Lime  stone.  9. 
Clay.  8.  Sand- 
stones. 7.  Sandy 
clays.  6.  Lime- 
stone with  part- 
ing of  shale.  5. 
Shale.  4.  Lime- 
stone. 3.  Shale 
with  cement- 
stone  passing 
down  into  sand- 
stone (2),  which 
graduates  into 
fi  n  e  conglomer- 
ate (1). 


182 


GEOLOGY 


deed,  it  cannot  be  adequately  followed  unless,  by  the  proper  use 
of  the  imagination,  the  former  conditions  of  the  earth's  surface 
are  vividly  realised. 

The  thinnest  layers  of  a  stratified  rock  form  lamina,  such  as 
the  thin  paper-like  leaves  into  which  shale  can  be  split.  A  num- 
ber of  laminaB  may  be  united  in  a  stratum  or  bed  which  may 
vary  from  less  than  an  inch  to  several  feet  or  yards  in  thickness. 
It  is  only  the  finer  kinds  of  sedimentary  rock  that,  as  a  rule,  are 
laminated.  In  other  cases  a  stratum  or  bed  is  the  thinnest  sub- 
division ;  it  can  usually  be  separated  easily  from  those  above  and 
below  it,  and  it  may  generally  be  regarded  as  marking  one  con- 
tinued phase  of  deposit,  while  the  break  between  it  and  the  next 
bed  above  or  below  probably  denotes  an  interruption  of  the  de- 
posit. The  study  of  the  relations  of  strata  to  each  other  is  called 
Stratigraphy. 


Fig.  81. —  False-bedded  sandstone. 

Layers  of  deposit  usually  lie  parallel  with  each  other,  their 
flat  surfaces  marking  the  general  floor  of  the  water  at  the  time 
of  their  formation  (Figs.  79,  80).  But  sometimes  a  series  of 
layers  may  be  found  inclined  at  various  angles  to  what  was  ob- 
viously the  original  general  plane  of  deposition.  In  Fig.  81, 
for  example,  a  series  of  strata  is  presented,  which  are  dis- 
tinguished by  a  diagonal  lamination.  This  is  known  as  False 
bedding  or  Current-bedding.  As  before  explained  in  Chapter  III 
it  has  been  caused  by  the  pushing  of  layers  of  sediment  over 
the  advancing  front  of  a  stratum,  and  may  be  compared  to  the 


SEDIMENTARY  ROCKS  183 

oblique  bedding  often  to  be  seen  in  an  earthwork,  such  as  a 
railway  embankment,  the  upper  surface  of  which  may  be  in  a 
general  sense  parallel  with  the  flat  bottom  of  the  valley,  while 
the  successive  layers  of  which  the  mound  is  made,  are  inclined  at 
angles  of  30°  or  more.  False  bedding  is  interesting  as  afford- 
ing some  indication  of  the  nature  and  direction  of  the  currents 
by  which  sediment  has  been  transported. 

PROOFS  or  FORMER  SHORES. —  Along  the  margin  of  the  sea, 
of  lakes,  and  of  rivers,  several  interesting  kinds  of  markings 
may  be  seen  impressed  on  surfaces  of  sand  or  mud  from  which 
the  water  has  retired.  Every  one  who  has  walked  on  a  tidal 


Fig.  82. —  Ripple-marked  surface  of  sandstone. 

sea-beach  is  familiar  with  the  Ripple-marks  left  by  the  retreating 
tide  upon  the  bare  sands.  They  are  produced  by  the  oscillation 
of  the  water  driven  into  movement  by  wind  playing  over  its 
surface.  They  are  usually  effaced  by  the  next  advancing  tide; 
hence,  out  of  the  same  sand  new  sets  of  ripple-marks  are  made 
by  each  tide.  But  we  can  understand  that  now  and  then,  under 
peculiarly  favourable  conditions,  the  markings  may  not  be  de- 
stroyed. If,  for  instance,  they  were  made  in  a  kind  of  muddy 
sand,  which,  in  the  interval  between  two  tides  and  under  a  strong 
sun,  could  become  hard  and  coherent  on  the  surface,  and  if  the 
next  tide  advanced  so  quietly  as  not  to  disturb  them,  but  to  lay 


184  GEOLOGY 

down  upon  them  a  fresh  layer  of  sand  or  mud,  they  might  be 
covered  up  and  preserved.  They  would  then  remain  as  a  me- 
morial of  the  shallow  rippling  water  and  bare  sandy  shore  where 
they  had  been  |ormed. 

Now  evidence  of  this  kind  regarding  the  conditions  of  deposi- 
tion occur  abundantly  among  sedimentary  rocks  (Fig.  82).  Eip- 
ple-marked  surfaces  may  be  traced  one  over  another  for  many 
hundred  feet  in  a  thick  series  of  sandstones.  They  bring  clearly 
to  the  mind  that  the  strata  on  which  they  lie  were  accumulated 
in  shallow  water,  or  along  beaches  that  were  often  laid  dry. 

LAND-SURFACES. —  Other  traces  of  exposure  to  the  air  may  be 
noticed,  where  ripple-mark  is  abundant,  in  what  are  termed  Sun- 
cracks,  Foot-prints,  and  Rain-prints.  Those  who  have  observed 


Fig.  83. —  Cast  of  a  sun-cracked  surface  preserved  in  the  next  succeeding 
layer  of  sediment. 

what  takes  place  in  muddy  places  during  dry  weather  will  re- 
member that,  as  the  mud  dries  and  contracts  it  splits  up  into  a 
network  of  cracks;  and  that,  on  its  hardened  surface,  it  retains 
impressions  of  the  feet  of  birds  or  of  insects  that  may  have 
walked  over  it  while  still  soft.  The  geological  history  recorded 
at  such  places  cannot  be  mistaken;  first,  the  rainy  period,  with 
the  rush  of  muddy  water  down  the  slopes  and  the  formation  of 
pools  in  which  the  mud  is  allowed  to  settle;  then  the  season  of 
warm  weather  when  the  pools  gradually  dry  up  and  birds  seek 
their  edges  to  drink.  If  by  any  means  a  layer  of  sediment  could 
be  laid  down  upon  one  of  these  desiccated  basins  so  gently  as  not 
to  efface  its  peculiar  markings,  the  cracked  surface  of  mud,  with 


SEDIMENTARY  ROCKS  185 

its  footprints,  would  contain  a  perfectly  intelligible  record  of  the 
changes  which  it  had  witnessed  (Fig.  83). 

Now  surfaces  of  this  kind  abound  among  the  sedimentary 
rocks  of  the  earth's  crust.  They  are  found  upon  strata  which, 
from  the  presence  of  marine  organic  remains  in  them,  were  cer- 
tainly deposited  under  the  sea.  But  these  strata  cannot  have 
accumulated  in  deep  water;  they  must  have  been  formed  along 
flat  shores,  where  the  sheets  of  sand  and  mud  were  liable  from 
time  to  time  to  be  laid  bare  to  the  sun  and  wind,  where  animals 
of  various  kinds  left  their  footmarks  or  trails  on  the  still  soft 
sediment,  where  the  evaporation  and  desiccation  were  so  rapid 


Fig.  84. —  Rain-prints  on  fine  mud. 

as  to  cause  the  exposed  mud  to  harden  on  the  surface  and  to 
crack  up  into  irregular  polygonal  cakes,  and  where  the  next  suc- 
ceeding layers  of  sediment  were  deposit  el  so  gently  as  to  cover 
up  and  preserve  the  sun-cracked  surfaces. 

One  further  piece  of  evidence  to  indicate  land-surfaces,  or,  at 
least,  shore-surfaces,  in  a  series  of  aqueous  sedimentary  strata,  is 
that  furnished  by  Eain-prints.  A  brief  shower  of  rain  leaves 
upon  a  smooth  surface  of  fine  sand  or  mud  a  series  of  small  pits, 
each  of  which  is  the  imprint  of  a  descending  raindrop  (Fig.  84). 
Where  this  takes  place  along  the  edge  of  a  muddy  pool  which  is 
rapidly  being  dried  up,  the  prints  of  the  drops  may  remain  quite 
distinct  on  the  hardened  surface  of  mud.  And  here,  again,  we 
can  suppose  that  if  another  layer  of  mud  were  gently  deposited 


186 


GEOLOGY 


above  this  surface  the  rain-prints  would  be  sealed  up  and  pre- 
served. We  might  even  be  able  to  tell  from  what  quarter  the 
wind  blew  that  brought  the  rain-cloud.  If,  for  example,  the 
rain-prints  were  ridged  up  on  one  side  in  one  general  direction 
this  would  show  that  the  shower  fell  aslant  and  with  some  force, 
and  that  the  side  on  which  the  mud  round  the  imprints  was 
forced  up  was  that  towards  which  the  rain  was  driven.  Such 
indications  of  ancient  weather  may  here  and  there  be  detected 
among  stratified  rocks. 

CONCRETIONS. —  Another  original  characteristic  of  many  sedi- 
mentary rocks  is  a  concretionary  structure,  particularly  observ- 


Fig.  85. — Vertical  trees  (Sigillaria)  in  sandstone,  Swansea  (Logan). 

able  in  clays,  limestones,  and  ironstones.  In  many  cases,  the 
concretions  have  gathered  round  some  fragment  of  a  plant  or  an 
animal.  Clay-ironstone  and  impure  limestone  have  been  aggre- 
gated into  spherical  or  elliptical  forms  (septaria),  which  are  of 
frequent  occurrence  in  clay  or  shale  (Figs.  61,  65).  Flint  has 
also  gathered  round  some  organic  nucleus,  which  it  has  often 
entirely  replaced.  But  many  concretions  may  be  found  where 
no  organic  fragment  as  a  starting-point  can  be  detected.  Some 
of  the  most  curious  are  the  so-called  Fairy-stones  (Fig.  64), 


SEDIMENTARY  ROCKS  187 

found  in  alluvial  clays,  with  so  many  imitative  shapes,  which 
have  been  popularly  supposed  to  be  works  of  human  or  even  pre- 
ternatural construction.  They  have  probably  been  produced  by 
the  irregular  cementing  of  clay,  owing  to  the  spread  of  car- 
bonate of  lime  through  it,  carried  down  by  permeating  water. 
Some  of  the  most  extraordinary  concretionary  masses  are  to  be 
seen  in  certain  magnesian  limestones,  which  appear  to  be  built 
up  of  petrified  lumps  of  coral,  bunches  of  grapes,  cannon-balls, 
and  other  objects  (Fig.  75).  In  reality,  all  these  diversified 
figures  are  due  to  the  irregularly  varied  way  in  which  a  concre- 
tionary structure  has  been  developed  in  the  limestone. 

ASSOCIATION  AND  ALTERNATION  OF  STRATA. —  Certain  kinds 
of  sedimentary  rocks  are  apt  to  occur  together  to  the  exclusion  of 
others.  This  association  depends  on  the  circumstances  of  depo- 
sition. Ironstone  concretions,  for  example,  are  much  more  fre- 
quent among  clays  or  shales  than  in  any  other  strata,  because  it 
was  during  the  deposit  of  fine  mud  with  abundant  decomposing 
organic  matter  that  the  most  favourable  conditions  were  sup- 
plied for  the  precipitation  of  carbonate  of  iron.  Clays  and  lime- 
stones frequently  alternate,  as  also  do  sandstones  and  conglomer- 
ates, because  the  circumstances  of  deposition  were  somewhat 
alike,  (see  Fig.  80).  But  we  need  not  expect  to  encounter  a  bed 
of  coarse  conglomerate  in  a  group  of  fine  clays,  for  the  current 
that  was  strong  enough  to  sweep  along  the  stones  of  the  con- 
glomerate was  too  powerful  to  allow  the  fine  silt  to  lie  undis- 
turbed. For  a  similar  reason,  we  should  be  surprised  to  meet 
with  a  layer  of  well-stratified  shale  in  a  mass  of  conglomerate. 
The  agitated  water  in  which  these  coarse  materials  were  heaped 
up  would  have  swept  away  any  fine  sediment  and  prevented  it 
from  being  deposited.  In  all  cases,  the  manner  in  which  the 
different  kinds  of  sediment  are  associated  with  each  other  leads 
us  back  directly  to  the  original  conditions  of  deposit,  and  is  only 
intelligible  in  proportion  as  these  conditions  are  clearly  realised. 

EELATIVE  AREAS  OF  STRATIFIED  ROCKS. —  Moreover,  some 
kinds  of  sedimentary  material  must  obviously  spread  over  wider 
areas  than  others.  The  coarse  gravel  and  shingle  of  the  present 
beach  do  not  extend  far  seawards ;  they  are  confined  to  the  mar- 
gin of  the  land.  Sand  covers  the  sea-floor  over  a  wider  area; 
and  beyond  the  limits  of  the  sand,  in  the  deeper  and  stiller  water, 
mud  is  allowed  to  accumulate.  Eoughly  speaking,  therefore,  the 


188  GEOLOGY 

area  of  the  distribution  of  sediment  is  in  inverse  proportion  to 
the  coarseness  of  the  materials.  The  same  law  has  regulated  the 
accumulation  of  detritus  from  early  geological  time.  Coarse 
conglomerates,  which  represent  ancient  shingles  and  gravels 
thicken  and  thin  out  rapidly,  and  do  not  usually  cover  a  large 
area,  though  they  may  sometimes  be  traced  for  long  distances 
in  the  direction  probably  of  the  original  coast  or  line  of  heaping 
up  of  the  shingle.  They  pass  laterally  and  vertically  into  grit 
and  sandstones  which  have  a  much  wider  distribution,  and  then 
again  shade  off  into  clays  and  shales  that  range  also  over  large 
areas. 

CHRONOLOGICAL  VALUE  OP  STRATA. — No  clue  has  yet  been 
found  to  determine  the  length  of  time  required  for  the  accumu- 
lation of  a  stratum  or  group  of  strata ;  but  some  indications  are 
afforded  of  relative  lapse  of  time.  Here  and  there,  for  instance, 
where  trunks  of  trees  are  met  with  standing  in  their  positions 
of  growth,  but  imbedded  in  solid  sandstone  (Fig.  85).  These 
stems,  sometimes  20  feet  or  more  in  height,  prove  that  a 
mass  of  sand  of  that  depth  must  have  been  accumulated  around 
them  before  they  had  time  to  decay.  We  know  little  about  the 
durability  of  the  submerged  trees;  but  they  probably  could  not 
have  lasted  long  unless  covered  up  by  sediment ;  so  that  the  mass 
of  strata  in  which  they  are  enclosed  may  be  supposed  to  have 
been  accumulated  within  a  few  years.  The  nature  of  the  mate- 
rial composing  sedimentary  rocks  may  likewise  furnish  indica- 
tions of  relative  rate  of  deposition.  Thus  finely  laminated  clays 
were  evidently  deposited  with  extreme  slowness.  Beds  of  lime- 
stone, composed  of  the  crowded  remains  of  successive  genera- 
tions of  marine  creatures,  must  also  have  required  prolonged 
periods  of  time  for  their  growth.  On  the  other  hand,  thick  beds 
of  sandstone  preserving  great  uniformity  of  characters  may  not 
improbably  have  been  laid  down  with  comparative  rapidity. 

A  reliable  inference  can  be  drawn  from  the  mere  thicknesses 
of  strata  as  to  the  lapse  of  time  which  they  represent.  A  mass 
of  sandstone  20  feet  thick  may  have  accumulated  round  a  sub- 
merged tree  in  a  few  years.  On  the  other  hand,  a  corresponding 
depth  of  fine  laminated  clay  may  have  required  tenfold  more  time 
for  its  deposition.  But  the  same  thickness  of  rock  composed  of 
alternations  of  shale  and  limestone  might  represent  a  still  longer 
period.  For  it  is  obvious  that  the  change  from  one  kind  of 


SEDIMENTARY  ROCKS  189 

sediment  to  another  must  often  have  been  brought  about  by  an 
extremely  gradual  modification  of  the  geography  of  the  region 
from  which  the  supply  of  sediment  was  derived.  Hence  the 
interval  between  two  beds  or  groups  of  beds,  differing  much  from 
each  other  in  mineral  composition,  may  have  been  considerably 
longer  than  the  time  required  for  the  actual  deposition  of  the 
strata  of  either  or  both  beds  or  groups  of  beds. 

On  any  probable  estimate,  the  deposition  of  sedimentary  rocks 
to  a  depth  of  many  thousand  feet  and  over  areas  many  thousands 
of  square  miles  in  extent,  must  have  demanded  enormous  periods 
of  time.  Side  by  side  with  the  growth  of  mechanical  sediments, 
there  must  have  been  a  corresponding  wasting  of  land.  Every 
bed  of  conglomerate,  sand,  or  mud  represents  at  least  an  equiva- 


Fig.  86. —  Hills  formed  out  of  horizontal  sedimentary  rocks. 

lent  amount  of  rock  worn  away  from  the  land  and  transported 
as  sediment  to  the  floor  of  the  sea.  During  such  prolonged 
ages  as  these  changes  required,  there  was  ample  time  for  the 
outburst  of  many  successive  volcanoes,  for  the  passage  of  many 
earthquake-shocks,  and  for  the  subsidence  or  upheaval  of  many 
parts  of  the  earth's  crust. 

PROOFS  OF  SUBSIDENCE. —  A  mass  of  sedimentary  material  of 
great  thickness  which,  from  the  remains  of  sun-cracks  and  other 
evidence,  was  obviously  deposited  in  shallow  water  near  land  can 
only  have  been  accumulated  on  an  area  that  was  gradually  sink- 
ing. Suppose,  for  instance,  that  a  hill  formed  out  of  such  strata 
rises  a  thousand  feet  above  the  valley  at  its  foot  (Fig.  86),  and 
that  proofs  of  deposition  in  shallow  water  can  be  detected  from 
the  lowest  beds  all  the  way  up  to  the  highest.  The  lowest  beds 
having  once  been  close  to  the  surface,  as  shown  by  the  sun-cracks 


190  GEOLOGY 

and  other  evidence,  could  only  be  covered  with  hundreds  of  feet 
of  similar  strata  by  a  gradual  sinking  of  the  ground,  during 
which  fresh  sediment  was  poured  in,  so  that,  although  the  origi- 
nal bottom  sank  a  thousand  feet,  the  water  may  never  have  be- 
come sensibly  deeper,  the  rate  of  deposit  of  sediment  having,  on 
the  whole,  kept  pace  with  that  of  the  subsidence. 

OVERLAP. —  During  such  tranquil  movements,  as  the  area  of 
land  lessens  and  that  of  the  sea  increases,  the  later  sedimentary 
accumulations  must  needs  extend  beyond  the  limits  of  the  older 
ones.  Suppose,  for  instance,  that  such  a  sloping  land-surface  as 
that  represented  in  the  section  (x,  Fig  87)  were  slowly  to  subside 


Fig.   87. —  Section  of  overlap. 


beneath  the  sea,  the  first-formed  strata  (a)  will  be  covered  and 
overlapped  by  the  next  series  ( b ) ,  and  these  in  turn,  as  the  sea- 
floor  sinks,  will  be  similarly  concealed  by  the  following  group 
(c) .  This  structure,  termed  Overlap,  may  usually  be  regarded  as 
evidence  of  a  gentle  subsidence  of  the  area  of  deposit. 
.  CONFORMABILITY,  TlNCONFORMABiLiTY. —  When  stratified  de- 
posits are  laid  down  regularly  and  continuously  upon  each  other, 
with  no  interruption  of  their  generally  level  position,  they  are 
said  to  be  conformable.  In  the  section  Fig.  80,  for  instance,  the 
series  of  sediments  there  represented  has  evidently  been  deposited 
under  the  same  general  conditions.  The  nature  of  the  sediment 
has  of  course  varied  from  time  to  time;  limestones,  shales,  and 
sandstones  have  alternated  with  each  other ;  but  there  has  been  no 
marked  interruption  or  disturbance  in  their  sequence.  Suppose, 
however,  that  owing  to  subterranean  movements,  a  series  of  rocks 
(a  in  Fig.  88)  is  shifted  from  its  original  position  and  after  be- 
ing uplifted,  is  exposed  to  the  wearing  action  of  the  sea,  rivers, 
air,  rain,  frosts,  and  the  other  agents  concerned  in  the  degrada- 
tion of  the  surface  of  the  land.  If  a  new  series  of  deposits  (&) 
is  laid  down  upon  the  denuded  edges  of  these  rocks,  the  bedding 
of  the  whole  will  not  be  continuous.  The  younger  strata  will  rest 
successively  upon  different  parts  of  the  older  group,  or,  in  other 
words,  will  be  unconformable.  Such  a  relation  or  unconform- 


SEDIMENTARY  ROCKS 


191 


ability  (unconformity)  implies  a  terrestrial  disturbance,  and 
usually  also  the  lapse  of  a  long  interval  of  time  between  the  re- 
spective periods  of  the  older  and  younger  rocks  during  which 
denudation  of  the  older  strata  took  place.  It  serves  to  mark 
one  of  the  breaks  or  gaps  in  geological  history.  Unconformabili- 
ties  differ  much  from  each  other  in  regard  to  the  length  of  in- 
terval which  they  denote.  In  some  cases,  the  blank  may  be  of 
comparatively  slight  moment ;  in  others,  it  is  so  vast  as  to  include 
the  greater  part  of  the  time  represented  by  the  stratified  rocks  of 
the  earth's  crust. 

By  means  of  unconformabilities  the  different  ages  of  mountain- 
chains  are  determined.  If,  for  example,  a  mountain  showed  the 
structure  represented  in  Fig.  88,  its  upheaval  must  obviously  have 
taken  place  between  the  deposition  of  the  two  series  of  rocks. 
Suppose  the  series  a  to  represent  Lower  Silurian,  and  &  Carboni- 
ferous rocks,  the  date  of  the  mountain  would  be  between  the 


Fig.  88. —  Unconformability. 

Lower  Silurian  and  Carboniferous  periods.  If,  in  another  moun- 
tain, series  b  were  unconformably  overlain  by  a  younger  series, 
say  of  Jurassic  age,  this  mountain  would  thereby  be  shown  to 
have  undergone  a  subsequent  uplift  in  the  long  interval  between 
the  Carboniferous  and  the  Jurassic  periods. 

SUMMARY. —  In  this  Lesson  some  of  the  more  characteristic 
original  features  of  sedimentary  rocks  have  been  considered.  Of 
these  features,  one  of  the  most  distinctive  is  the  arrangement  into 
layers  of  beds,  each  of  which  is  the  record  of  a  portion  of  geologi- 
cal history,  the  oldest  being  below  and  the  youngest  above.  The 
smallest  subdivision  of  these  records  is  a  lamina  or  thin  leaf, 
such  as  those  into  which  shales  may  be  split.  A  stratum  or  bed, 
which  may  contain  many  laminae  or  none,  is  a  thicker  layer 
separable  with  more  or  less  ease  from  those  below  and  above  it. 
Though  strata  lie  on  the  whole  parallel  with  each  other,  they 
often  show  oblique  current-bedding,  especially  in  sandstones. 


192  GEOLOGY 

Traces  of  shore-lines  and  of  surfaces  laid  bare  by  the  retirement 
of  the  water  in  which  they  were  deposited,  are  found  in  sun- 
cracks,  rain-pittings,  and  footprints.  Not  infrequently,  instead 
of  being  evenly  spread  out  in  layers,  the  sedimentary  material 
has  been  aggregated  into  variously-shaped  concretions.  Certain 
kinds  of  sedimentary  rocks  are  apt  to  occur  together,  such  as  clays 
and  limestones,  clay-ironstones  and  shales,  coals  and  fire-clays; 
because  the  conditions  under  which  they  were  respectively  de- 
posited were  on  the  whole  similar.  As  a  rule,  the  finer  the 
detritus,  the  wider  the  area  over  which  it  is  spread ;  hence  clays 
generally  cover  wider  tracts  than  conglomerates.  No  inference 
can  safely  be  drawn  from  the  relative  thickness  of  strata  as  to  the 
length  of  time  which  they  respectively  represent ;  they  must  vary 
widely  in  this  respect,  and  it  is  quite  conceivable  that,  in  many 
cases,  the  interval  of  time  between  the  deposition  of  two  succes- 
sive beds  of  very  different  character  and  composition  may  have 
been  actually  longer  than  the  period  required  for  the  deposition 
of  two  beds.  A  thick  series  of  sedimentary  deposits  usually  in- 
dicates that  the  sea-bottom  on  which  it  was  laid  down  was  slowly 
sinking.  In  subsiding,  the  later  deposits  spread  beyond  the 
limits  of  the  earlier  ones,  and  thus  present  what  is  called  an 
overlap.  Where  they  have  been  laid  down  continuously  one  upon 
another  they  are  said  to  be  conformable;  where  one  group  has 
been  deposited  on  the  disturbed  and  worn  edges  of  an  older  series 
the  two  are  unconf  ormable  to  each  other. 


SEDIMENTARY  ROCKS  AFTER  FORMATION  193 


CHAPTER  XIII. 

SEDIMENTARY  ROCKS  AFTER  FORMATION. 

AFTER  their  deposition  sedimentary  materials  have  under- 
gone various  changes  before  assuming  the  aspect  which 
they  now  wear. 

CONSOLIDATION. —  The  most  obvious  of  these  changes  is  that, 
instead  of  consisting  of  loose  materials,  gravel,  sand,  mud  and  so 
on,  they  are  now  hard  stone.  This  consolidation  has  sometimes 
been  the  result  of  mere  pressure.  As  bed  was  piled  over  bed, 
those  at  the  bottom  would  gradually  be  more  and  more  com- 
pressed by  the  increasing  weight  of  those  that  were  laid  down 
upon  them,  the  water  would  be  squeezed  out,  and  any  tendency 
which  the  particles  might  have  to  cohere  would  promote  the  con- 
solidation of  the  mass.  Mud,  for  example,  might  in  this  way  be 
converted  into  clay,  and  clay  in  turn  might  be  pressed  into  mud- 
stone  or  shale.  But  besides  cohesion  from  the  pressure  of  over- 
lying masses,  sedimentary  matter  has  often  been  bound  together 
by  some  kind  of  cement,  either  originally  deposited  with  it  or 
subsequently  introduced  by  permeating  water.  Among  natural 
cements,  the  most  common  are  silica,  carbonate  of  lime  and  per- 
oxide of  iron.  In  a  red  sandstone,  for  example,  the  quartz-grains 
may  be  observed  to  be  coated  over  with  earthy  iron  peroxide, 
which  serves  to  unite  them  together  into  a  more  or  less  coherent 
stone.  The  effect  of  weathering  is  not  infrequently  to  remove 
the  binding  cement  and  thereby  to  allow  the  stone  to  return  to 
its  original  condition  of  loose  sediment. 

JOINTS. —  Next  to  their  consolidation  into  stone,  the  most  com- 
mon change  which  has  affected  sedimentary  rocks  is  the  produc- 
tion in  them  of  a  series  of  divisional  planes  or  fractures  termed 
Joints.  Except  in  loose  incoherent  materials,  this  structure  is 
hardly  ever  absent.  In  any  ordinary  quarry  of  sandstone,  lime- 
stone, or  other  sedimentary  rock,  or  along  a  natural  cliff  of  the 


194  GEOLOGY 

same  materials,  a  little  attentive  observation  will  show  that  the 
bare  wall  of  rock  forming  the  back  of  the  quarry  or  the  face  of 
the  cliff  has  been  determined  by  one  or  more  natural  fissures 
in  the  stone,  and  that  there  are  other  fissures  running  parallel 
with  it  through  every  outstanding  buttress  of  rock.  Moreover, 
we  may  observe  that  these  vertical  or  highly  inclined  lines  of 
fissure  are  cut  across  by  others,  more"  or  less  nearly  at  a  right 
angle,  and  that  the  sides  of  the  buttresses  have  been  defined 
by  these  transverse  lines,  just  as  the  main  face  of  rock  has  been 
formed  by  the  first  set.  Such  lines  of  division  are  Joints.  In 
close-grained  stone  they  may  be  imperceptible  until  it  is  quarried 
or  broken,  when  they  reveal  themselves  as  sharply  defined,  nearly 
vertical  fractures,  along  which  the  stone  splits.  There  are  usually 


Fig.  89. —  Joints  in  a  stratified  rock. 

at  least  two  series  of  joints  crossing  each  other  at  right  angles 
or  obliquely,  whereby  a  rock  is  divided  into  quadrangular  blocks. 
In  the  accompanying  diagram  (Fig.  89)  a  group  of  stratified 
rocks  is  seen  to  be  traversed  by  two  sets  of  joints,  one  of  which 
(dip- joints,  "cutters"  of  the  quarrymen)  defines  the  faces 
that  are  in  shadow,  the  other  (strike- joints,  "backs"  of  the 
quarrymen)  those  that  are  in  light.  By  help  of  these  divisional 
planes,  it  is  possible  to  obtain  large  blocks  of  stone  for  building 
purposes.  The  art  of  the  quarryman,  indeed,  largely  consists  of 
taking  advantage  of  these  natural  lines  of  fracture,  so  as  to 
obtain  his  materials  with  the  least  expenditure  of  time  and 
labour,  and  in  large  masses.  In  nature  also  the  existence  of 
joints  is  a  fact  of  the  highest  importance.  Eeference  has  already 


SEDIMENTARY  ROCKS  AFTER  FORMATION  195 

been  made  to  the  way  in  which  they  afford  a  passage  for  the 
descent  of  water  from  the  surface.  It  is  in  great  measure  along 
joints  that  the  underground  circulation  of  water  is  conducted. 
At  the  surface,  too,  where  rocks  yield  to  the  decomposing  influ- 
ence of  the  weather,  it  is  by  their  joints  that  they  are  chiefly 
split  up.  Along  these  convenient  planes  of  division,  rain-water 
trickles  and  freezes;  the  walls  of  the  joints  are  separated,  and 
the  space  between  them  is  slowly  widened,  until  in  the  end  it 
opens  into  yawning  rents,  and  portions  of  a  cliff  are  overbalanced 
and  fall,  while  detached  pinnacles  are  here  and  there  isolated. 
The  picturesqueness  of  the  scenery  of  stratified  rock  is,  in  great 
measure,  dependent  upon  the  influence  of  joints  in  promoting 
their  dislocation  and  disintegration  by  air,  rain,  and  frost. 

In  many  cases,  joints  may  be  due  to  contraction.  A  mass  of 
sand  or  mud,  as  it  loses  water  and  as  its  particles  are  more  firmly 
united  to  each  other,  gradually  occupies  less  room  than  at  first. 
In  consequence  of  the  contraction  strains  are  set  up  in  the  stone, 
and  relief  from  these  is  eventually  found  in  a  system  of  cracks 
or  fissures.  In  other  instances,  joints  have  been  produced  by 
the  compression  or  torsion  to  which  large  masses  of  rock  have 
been  exposed  during  movements  of  the  earth's  crust. 

ORIGINAL  HORIZONTALITY. —  As  laid  down  upon  the  margin 
or  floor  of  the  sea.,  on  the  bottoms  of  lakes,  and  on  the  beds  or 
alluvial  plains  of  rivers,  sedimentary  accumulations  are  in  general 
nearly  flat.  They  slope  gently,  indeed,  seawards  from  a  shelving 
shore,  and  they  gather  at  steeper  angles  on  slopes  of  debris  at 
the  foot  of  cliffs,  or  down  the  sides  of  mountains.  But,  taken 
as  a  whole,  and  over  wide  areas,  their  original  position  is  not  far 
removed  from  the  horizontal.  If  we  turn,  however,  to  the  sedi- 
mentary rocks  that  form  so  large  a  part  of  the  earth's  crust, 
and  so  much  of  the  dry  land,  we  find  that  although  originally 
deposited  for  the  most  part  over  the  sea-bottom,  they  are  now 
inclined  at  all  angles,  and  even  sometimes  stand  on  end.  Such 
situations,  in  which  their  deposition  could  never  have  taken 
place,  show  that  they  have  been  disturbed.  Not  only  have  they 
been  upraised  into  land,  but  they  have  been  tilted  unequally, 
some  parts  rising  or  sinking  much  more  than  others. 

DIP. —  The  inclination  of  bedded  rocks  from  the  horizon  is 
called  their  Dip.  The  amount  of  dip  is  reckoned  from  the  plane 
of  the  horizon.  A  face  of  rock  standing  up  vertically  above  that 


196 


GEOLOGY 


plane  is  said  to  be  at  90°,  while  midway  between  that  position 
and  horizontally  it  lies  at  an  inclination  of  45°.  The  angle  of 
dip  is  accurately  measured  with  an  instrument  called  a  Clino- 
meter, of  which  there  are  various  forms.  One  of  the  simplest 
kinds  is  a  brass  half-circle  graduated  into  90°  on  each  side  of 
the  vertical,  on  which  a  pendulum  is  hung  as  in  Fig.  91.  The 


/   <:•     A 

/ .  My,™ 


Fig.  90. —  Dip  and  Strike.     The  arrow  shows  the  direction  of  dip;  the 
line  s  s  marks  the  strike. 

instrument  is  held  between  the  eye  and  the  angle  to  be  measured, 
and  the  upper  edge  is  made  to  coincide  with  the  line  of  the 
inclined  rock.  The  pendulum,  remaining  vertical,  points  to  the 
angle  of  inclination  from  the  horizon.  A  little  practice,  how- 
ever, enables  an  observer  to  estimate  the  amount  of  dip  by  the 
eye  with  sufficient  accuracy  for  most  purposes.  The  direction  of 
dip  is  the  point  of  the  compass  toward  which  a  stratum  is 
inclined  (shown  by  the  arrow  in  Fig.  90),  and  is  best  ascer- 


Fig.  91. —  Clinometer. 

tained  with  a  magnetic  compass.  But  here  again  a  little  experi- 
ence in  judging  of  the  quarters  of  the  sky  without  an  instrument 
will  usually  enable  us  to  tell  the  direction  of  dip  with  as  much 
precision  as  may  be  required. 

STRIKE. —  A  mathematical  line  running  at  a  right  angle  to 
the  direction  of  dip  is  called  the  Strike  (  s  s  in  Figs.  90,  92). 
Where  a  series  of  strata  dips  due  north  or  due  south  the  strike 


SEDIMENTARY  ROCKS  AFTER  FORMATION  197 

is  east  and  west;  but  the  direction  of  strike  changes  with  that 
of  the  dip.  Suppose,  for  example,  that  certain  strata  dip  due 
east,  then  veer  round  by  south-east  to  south,  and  so  on  by  west 
and  north,  back  to  east  again.  The  strike  following  this  change 
would  describe  a  circle.  In  fact,  the  beds  would  be  included  in 
a  basin-shaped  or  dome-shaped  arrangement  and  the  strike  would 
be  the  lip  of  the  basin  or  rim  of  the  truncated  dome.  Though 
the  dip  may  slightly  vary  from  place  to  place,  still,  if  it  remains 
in  the  same  general  direction  along  the  line  of  certain  strata, 
their  strike  is  on  the  whole  uniform. 

OUTCROP. —  The  actual  edge  presented  by  a  stratum  at  the 
surface  of  the  ground  is  called  its  Outcrop.  On  a  perfectly  level 
surface,  strike  and  outcrop  must  coincide;  but  as  ground  is 


P 

Fig.  92. —  Dip,  Strike,  and  Outcrop. 

seldom  quite  level  they  usually  diverge  from  each  other,  and  do 
so  the  more  in  proportion  to  the  lowness  of  angle  of  dip  and 
the  inequalities  of  the  ground.  This  may  be  illustrated  by  a 
diagram  such  as  that  given  in  Fig.  92,  which  represents  a  portion 
of  the  edge  of  a  table-land,  deeply  trenched  by  two  valleys  that 
discharge  their  waters  into  the  plain  below  (P).  The  arrows 
point  out  that  the  strata  dip  due  N.  at  5°.  On  the  level  plain, 
the  outcrop  and  the  strike  (s  s)  of  the  beds  are  coincident  and 
run  due  E.  and  W.  But  as  the  surface  rises  toward  the  high 
ground  and  the  deep  valleys,  the  outcrop  (o  o)  is  observed  to 
depart  mo^  and  more  from  the  strike  till  in  some  places  the 
two  lines  are  at  right  angles;  yet,  as  the  dip  remains  the  same, 


198 


GEOLOGY 


the  strike  is  likewise  unchanged,  the  sinuosities  of  the  outcrop 
being  entirely  due  to  the  irregularities  pf  the  surface  of  the 
ground. 

CUEVATURE. —  It  requires  no  long  observation  to  perceive  that 
in  being  tilted  from  their  original  more  or  less  level  positions, 
stratified  rocks  have  been  thrown  into  curves.  Suppose,  for 
instance,  that  in  walking  along  a  mile  of  coast-line,  where  all 
the  successive  strata  of  a  thick  series  are  exposed  to  view,  we 
should  observe  such  a  section  as  is  drawn  in  Fig.  93.  Beginning 


Fig.  93. —  Inclined  strata  shown  to  be  parts  of  curves. 

at  A,  we  find  the  beds  tilted  up  at  angles  of  70°  which  gradually 
lessen,  till  at  B  they  have  sunk  to  15°.  As  there  is  no  break  in 
the  series,  it  is  evident  that  the  lines  of  bedding  must  be  pro- 
longed downward,  and  must  once  have  been  continued  upward 
in  some  such  way  as  is  expressed  by  the  dotted  lines.  The 
visible  portion  which  is  here  shaded  must  thus  form  part  of 
a  great  curvature  of  the  rocks. 

But  the  actual  curvature  may  often  be  seen  on  coast-cliffs, 
ravines,  or  hillsides.  In  Fig.  94,  for  example,  a  simple  arch  is 
shown  from  the  Berwickshire  coast,  wherein  hard  beds  of  gray- 


SEDIMENTARY  ROCKS  AFTER  FORMATION 


199 


wacke  and  shale  have  been  folded.  Again,  in  Fig.  95,  the  reverse 
structure  is  exhibited,  beds  of  grit  and  slate  being  there  curved 
into  a  trough.  Where  rocks  dip  away  from  a  central  line  of 
axis  the  structure  is  known  as  an  Anticline ;  where,  on  the  other 
hand,  they  dip  towards  an  axis  it  is  called  a  Syncline.  In  Figs. 
94  and  95  these  two  structures  are  presented  on  so  small  a  scale 
as  to  be  visible  in  a  single  section.  More  usually,  however,  it 
is  only  by  observing  the  upturned  edges  of  strata  that  anticlines 
and  synclines  can  be  detected.  The  dark  part  of  Fig.  96  repre- 


Fig.  94. —  Curved  strata  (anticlinal  fold),  near  St.  Abb's  Head. 

sents  all  that  can  be  actually  seen ;  but  the  angles  and  direction 
of  dip  leave  no  doubt  that  if  we  could  restore  the  amount  of 
rock  which  has  here  been  worn  away  from  the  surface  of  the 
land,  the  present  truncated  ends  of  the  strata  would  be  pro- 
longed upward  in  some  such  way  as  is  indicated  by  the  dotted 
lines.  By  observations  of  this  truncation  of  strata  some  of  the 
most  interesting  and  important  evidence  is  obtained  of  the 
enormous  extent  to  which  the  land  has  been  reduced  by  the 
removal  of  solid  material  from  its  surface. 

PLICATION,    SHEARING. —  From   such   simple   curvatures   as 
those  depicted  in  the  foregoing  diagrams,  we  may  advance  to 


200  GEOLOGY 

more  complex  foldings,  wherein  the  solid  strata  have  been 
doubled  up  and  crumpled  together,  as  if  they  had  been  mere 
layers  of  carpet.  So  far  is  this  plication  sometimes  carried, 
that  the  lowest  rocks  are  brought  up  and  thrown  over  the  highest, 
the  more  yielding  materials  being  squeezed  into  the  most  intricate 
frillings  and  puckerings.  It  is  in  mountainous  regions,  where 
the  crust  of  the  earth  has  been  subjected  to  the  most  intense 
corrugation,  that  these  structures  are  best  seen.  We  can  form 
some  idea  of  the  gigantic  energy  of  the  earth-movements  that 
produced  them,  when  we  see  a  whole  mountain-range  made  up 
of  solid  limestones  or  sandstones  which  have  been  bent,  twisted, 


Fig.  95. —  Curved  strata  (synclinal  fold),  near  Banff. 

crumpled,  and  inverted,  as  we  might  crush  up  sheets  of  paper 
(Fig.  97). 

So  enormous  has  been  the  compression  produced  by  impor- 
tant movements  of  the  earth's  crust,  that  the  solid  rocks  have 
actually  been  squeezed  out  of  shape  or  have  undergone  a  process 
of  shearing.  The  amount  of  distortion  may  sometimes  be  meas- 
ured by  the  extent  to  which  shells  or  other  organic  remains  are 
pulled  out  in  the  direction  of  movement.  In  Fig.  98  the  proper 
shape  of  a  trilobite  (Angelina  Sedgwickii)  is  given,  and  along- 
side of  it  is  a  view  of  the  same  organism  which  has  been  elongated 


SEDIMENTARY  ROCKS  AFTER  FORMATION 


201 


by  the  distortion  of  the  mass  of  rock  in  which  it  lies.  Further 
results  of  shearing  will  be  immediately  referred  to  in  connection 
with  the  cleavage  and  metamorphism  of  rocks. 

CLEAVAGE. —  One  of  the  most  important  structures  developed 
by  the  great  compression  to  which  the  rocks  of  the  earth's  crust 


\ 


I 

Fig.  96. —  Anticlines  (a  a)  and  Synclines  (66). 

have  been  exposed  is  known  as  Cleavage.  The  minute  particles 
of  rocks,  being  usually  of  irregular  shapes,  have  been  compelled 
to  arrange  themselves  with  their  long  axes  perpendicular  to  the 
direction  of  pressure  during  the  interstitial  movements  conse- 
quent upon  intense  subterranean  compression.  Hence,  a  fissile 
tendency  has  been  imparted  to  a  rock,  which  will  now  split  into 
leaves  along  the  planes  of  rearrangement  of  the  particles.  This 
superinduced  tendency  to  split  into  parallel  leaves,  irrespective 


Fig.  97. —  Section  of  folded  and  crumpled  strata  forming  the  Grosse  Wind- 
galle  (10,482  feet),  Canton  Uri,  Switzerland,  showing  crumpled  and 
inverted  strata  (after  Heim). 

of  what  may  have  been  the  original  structure  of  the  rock,  con- 
stitutes cleavage.  It  is  well  developed  in  ordinary  roofing-slates. 
Though  the  leaves  or  plates  into  which  a  slate  splits  resemble 
those  in  a  shale,  they  have  no  necessary  relation  to  the  layers 
of  deposition  but  may  cross  them  at  any  angle.  In  Fig.  99,  for 
instance,  the  original  bedding  is  quite  distinct  and  shows  that 


202 


GEOLOGY 


the  strata  have  been  folded  by  a  force  acting  from  the  right  and 
left  of  the  section;  the  parallel  highly  inclined  lines  traversing 
the  folds  of  the  bedding  represent  the  planes  of  cleavage.  Where 


Fig.  98. —  Distortion  of  fossils  by  the  shearing  of  rocks;  (a)  a  Trilobite 
(Angelina  Sedgwickii)  distorted  by  shearing,  the  direction  of  move- 
ment indicated  by  the  arrows;  (6)  the  same  fossil  in  its  natural  form. 

the  material  is  of  exceedingly  fine  grain,  such  as  fine  consolidated 
mud  the  original  bedding  may  be  entirely  effaced  by  the  cleavage, 
and  the  rock  will,  only  split  along  the  cleavage-planes.  Indeed, 


Fig.  99. —  Curved  and  cleaved  rocks.  Coast  of  Wigtonshire.  The  fine 
parallel  oblique  lines  indicate  the  cleavage,  which  is  finer  in  the 
dark  shales  and  coarser  in  the  thicker  sandy  beds. 

the  finer  the  grain  of  a  rock,  the  more  perfect  may  be  its  cleavage, 
so  that  where  alternations  of  coarser  and  finer  sediment  have 
been  subjected  to  the  same  amount  of  compression  cleavage  may 
be  perfect  in  the  one  and  rudely  developed  in  the  other,  as 
is  indicated  in  Fig.  99. 


SEDIMENTARY  ROCKS  AFTER  FORMATION 


203 


Cleavage  may  be  regarded  as  one  of  the  first  stages  in  the 
mechanical  deformation  of  a  rock,  and  the  production  of  schistose 
metamorphism  (Chap.  XI) .  Besides  being  compressed  and  having 
its  component  particles  rearranged  in  definite  planes,  the  rock 
may  likewise  reveal  under  the  microscope  that  new  minerals, 
such  for  example  as  crystallites  or  minute  flakes  of  some  mica, 


Fig.  100. —  Examples  of  normal  Faults. 

have  been  developed  out  of  the  general  matrix,  as  may  be  seen 
in  common  roofing-slate.  By  increasing  stages  of  crystallisation 
we  trace  gradations  into  phyllites  and  mica-schists. 

DISLOCATION. — Another  important  structure  produced  in  rocks 
after  their  formation  is  Dislocation.  Not  only  have  they  been 
folded  by  the  great  movements  to  which  the  crust  of  the  earth 
has  been  subjected,  but  the  strain  upon  them  has  often  been 
so  great  that  they  have  snapped  across.  Such  ruptures  of  con- 


Fig.  101. —  Sections  to  show  the  relations  of  Plications  (a,  6)  to  reversed 

Faults  (c). 

tinuity  present  an  infinite  variety  in  the  position  of  the  rocks 
on  the  two  sides.  Sometimes  a  mere  fissure  has  been  caused, 
the  rocks  being  simply  cracked  across,  but  remaining  otherwise 
unchanged  in  their  relative  situations.  But,  in  the  great  majority 
of  instances,  one  or  both  of  the  walls  of  a  fissure  have  moved, 
producing  what  is  termed  a  FAULT.  Where  the  displacement 
has  been  small,  a  fault  may  appear  as  if  the  strata  had  been 
sharply  sliced  through,  shifted,  and  firmly  pressed  together  again 


204  GEOLOGY 

(a  in  Fig.  100).  Usually,  however,  they  have  not  only  been  cut, 
but  bent  or  crushed  on  one  or  both  siies  (6) ;  while  not  infre- 
quently the  line  of  fracture  is  represented  by  a  bank  of  broken 
and  crushed  material  (Fault-rock,  c).  The  fracture  is  seldom 
quite  vertical ;  almost  always  it  is  inclined  at  angles  varying  up 
to  70°  or  more  from  the  vertical.  In  by  far  the  largest  number 
of  faults,  the  inclination  of  the  plane  of  the  fissure,  or  what  is 
called  the  Hade  of  the  fault,  is  away  from  the  side  which  has 
risen  or  toward  that  which  has  sunk.  In  the  examples  given 
in  Fig.  100,  a,  ~b,  this  relation  is  expressed;  but  in  nature  it 
often  happens. that  the  beds  on  two  sides  of  a  fault  are  entirely 
different  (Fig.  100,  c),  and  consequently  that  the  side  of  up- 
throw or  downthrow  cannot  be  determined  by  the  identification 
of  the  two  severed  positions  of  the  same  bed.  But  if  the  hade 
of  the  fault  can  be  seen,  we  may  usually  be  confident  that  the 
strata  on  the  upper  or  hanging  side  belong  to  a  higher  part  of 
the  series  than  those  on  the  lower  side.  Faults  that  follow  this 
rule  (normal  faults)  are  by  far  the  most  frequent.  They  occur 
universally,  and  are  probably  for  the  most  part  caused  by  sub- 
sidence in  the  earth's  crust.  In  adjusting  themselves  to  the 
new  position  into  which  a  downward  movement  brings  them, 
rocks  must  often  be  subject  to  such  strains  that  their  limit  of 
elasticity  is  reached,  and  they  break  across,  one  portion  settling 
down  farther  than  the  part  next  to  it.  In  a  normal  fault,  the 
same  bed  can  never  be  cut  twice  by  a  vertical  line.. 

In  mountainous  districts,  however,  and  generally  where  the 
rocks  of  the  earth's  crust  have  been  disrupted  and  pushed  over 
each  other,  what  are  termed  reversed  faults  occur.  In  these,  the 
hade  slopes  in  the  direction  of  upthrow,  and  a  vertical  line  may 
cut  the  same  beds  twice  on  opposite  sides  of  the  fracture  (Fig. 
101).  Such  faults  may  be  observed  more  particularly  where 
strata  have  been  much  folded.  A  fold  may  be  seen  to  have 
snapped  asunder,  the  whole  being  pushed  over,  and  the  upper 
side  being  driven  forward  over  the  lower. 

The  amount  of  vertical  displacement  between  the  two  frac- 
tured ends  of  a  bed  is  called  the  Throw  of  a  fault.  In  Fig.  102, 
for  example,  where  bed  a  has  been  shifted  from  &  to  d,  a  vertical 
line  dropped  from  the  end  of  the  bed  at  &  to  the  level  of  the 
corresponding  part  of  the  bed  at  e  will  give  the  amount  of  the 
subsidence  of  d,  which  is  the  throw.  Faults  may  be  seen  with 


SEDIMENTARY  ROCKS  AFTER  FORMATION  205 

a  throw  of  less  than  an  inch  —  mere  local  cracks  and  trifling 
subsidences  in  a  mass  of  rock ;  in  others  the  throw  may  be  many 
thousand  feet.  Large  faults  often  bring  rocks  of  entirely  differ- 
ent characters  together,  as,  for  instance,  shales  against  lime- 
stones or  sandstones,  or  sedimentary  against  eruptive  rocks. 
Consequently  they  are  not  infrequently  marked  at  the  surface 
by  the  difference  between  the  form  of  ground  characteristic  of 
the  two  kinds  of  rock.  One  side,  perhaps,  rises  into  a  hilly  or 
undulating  region,  while  the  other  side  may  be  a  plain.  Com- 
paratively seldom  does  a  fault  make  itself  visible  as  a  line  of 
ravine  or  valley.  On  the  contrary,  most  faults  cut  across  valleys 
or  only  coincide  with  them  here  and  there.  They  run  in  straight 
or  wavy  lines  which,  where  the  amount  of  displacement  is  great, 
may  be  traced  for  many  miles.  The  Scottish  Highlands,  for 
example,  are  bounded  along  their  southern  margin  by  a  great 


Fig.  102. —  Throw  of  a  Fault. 

fault  which  places  a  thick  series  of  sandstones  and  conglomerates 
on  end  against  the  flanks  of  the  mountains.  This  fault  may 
be  traced  across  the  island  from  sea  to  sea  —  a  distance  of  fully 
120  miles,  and  by  bringing  two  distinct  kinds  of  rocks  next  each 
other  along  a  nearly  straight  line  it  has  given  rise  to  the  boun- 
dary between  Highland  and  Lowland  scenery  which,  in  some 
places,  is  so  singularly  abrupt. 

In  regions  of  the  most  intense  terrestrial  disturbance,  tracts 
of  rock  many  square  miles  in  area  and  hundreds  or  thousands  of 
feet  in  thickness,  have  been  torn  away  and  pushed  upward  and 
forward  until  they  have  come  to  rest  on  rocks  originally  much 
higher  in  geological  position.  Such  displaced  cakes  or  slices  of 
the  earth's  crust  sometimes  rest  upon  an  almost  horizontal  or 
gently  inclined  platform  of  undisturbed  materials.  Vertical  or 
contorted  strata  are  thus  placed  above  others  which  may  be  flat 
or  but  little  inclined.  The  plane  of  separation  between  the  moved 
and  unmoved  masses  is  really  a  dislocation,  but  to  distinguish 


206 


GEOLOGY 


it  from  faults,  which  are  generally  placed  at  steep  angles,  it  is 
called  a  Thrust-plane.  Structures  of  this  kind  on  a  colossal  scale 
are  traceable  for  about  100  miles  in  the  north-west  of  Scotland. 

METAMORPHISM. —  The  last  structure,  which  will  be  mentioned 
in  this  chapter  as  having  been  superinduced  upon  rocks,  is  con- 
nected with  the  movements  to  which  plication,  cleavage,  and 
reversed  faults  are  due.  So  enormous  has  been  the  energy  with 
which  these  movements  have  been  carried  on,  that  not  only  have 
the  rocks  been  crumpled,  ruptured,  and  pushed  over  each  other, 
but  they  have  undergone  such  intense  shearing  that  as  was 
pointed  out  in  Chap.XI,  their  original  structure  has  been  partially 
or  wholly  effaced.  They  have  been  so  crushed  that  their  com 


Fig.  103. —  Ordinary  un- 
altered red  sandstone, 
Keeshorn,  Ross-shire 
(magnified). 


Fig.  104. —  Sheared  red 
sandstone  forming  now 
a  micaceous  schist, 
Keeshorn,  Ross-shire 
(magnified). 


ponent  particles  have  been  reduced,  as  it  were,  to  powder,  and 
have  assumed  new  crystalline  arrangements  along  the  shearing- 
planes  or  surfaces  of  movement.  A  sandstone,  for  example, 
which  in-  its  ordinary  state  shows,  when  magnified,  such  a  struc- 
ture as  is  represented  in  Fig.  103,  when  it  has  come  within 
the  influence  of  this  crushing  process  has  its  grains  of  quartz, 
felspar,  and  other  materials  flattened  and  squeezed  against  each 
other  in  one  general  direction  as  in  cleavage',  while  out  of  the 
crushed  debris  a  good  deal  of  new  mica  has  been  developed. 
This  change  may  be  intensified  until  the  component  grains  are 
hardly  recognisable,  and  the  proportion  of  new  mica  has  so  in- 
creased that  the  rock  has  become  a  mica-schist.  Other  new 
minerals  such  as  garnet,  may  likewise  make  their  appearance, 
until  the  rock  assumes  an  entirely  crystalline  structure.  Such 
an  alteration  of  the  internal  structure  of  a  rock  is  known  as 


SEDIMENTARY  ROCKS  AFTER  FORMATION     207 

Metamorphism.  Where  the  change  arises  from  mechanical 
movements  combined  with  chemical  rearrangement,  it  usually 
affects  a  wide  district,  and  is  then  spoken  of  as  regional  meta- 
"jiorpliism. 

There  are  wide  regions  of  the  earth's  surface  where  schists  of 
various  kinds  form  the  prevailing  rock.  Whether  they  have  all 
been  produced  by  the  shearing  and  alteration  of  previously- 
formed  rocks  has  not  yet  been  determined.  But  that  a  large 
number  of  schists  are  truly  altered  or  metamorphosed  rocks 
admits  of  no  doubt.  Sandstones,  shales,  limestones,  quartzites, 
diorites,  syenites,  granites,  in  short,  any  old  form  of  rock  that 
has  come  within  the  crushing  and  shearing  movements  here 
referred  to  has  been  converted  into  schist.  The  gradation  be- 
tween the  unaltered  and  the  metamorphic  condition  can  often 
be  clearly  traced.  Granite,  by  crushing,  passes  into  gneiss, 
diorite  into  hornblende-schist,  sandstone  into  quartz-schist  or 
mica-schist,  and  so  on.  Even  where  it  is  no  longer  possible  to 
tell  what  the  original  nature  of  the  metamorphosed  material  may 
have  been,  there  is  usually  abundant  evidence  that  the  rock  has 
undergone  great  compression. 

SUMMARY. —  In  this  Lesson  attention  has  been  directed  to 
new  structures  produced  in  sedimentary  rocks  after  their  forma- 
tion. Beginning  with  the  simplest  and  most  universal  of  these, 
we  find  that  sediments  have  been  consolidated  into  stone,  partly 
by  pressure,  and  partly  by  some  kind  of  cement,  such  as  silica 
or  carbonate  of  lime.  In  the  process  of  consolidation  and  con- 
traction, they  have  been  traversed  by  systems  of  joints,  or  have 
had  these  subsequently  produced  by  the  torsion  accompanying 
movements  of  the  crust.  Though  at  first  nearly  flat  they  have 
by  these  movements  been  thrown  into  various  inclined  positions, 
and  more  especially  into  undulating  folds  or  more  complicated 
plication  and  puckering.  So  great  has  been  the  compression 
under  which  they  have  been  moved,  that  a  cleavage  has  been 
developed  in  them.  They  have  also  been  everywhere  more  or 
less  fractured,  the  dislocations  being  due  either  to  their  gradual 
subsidence  or  to  excessive  plication.  Their  most  complete  altera- 
tion is  seen  in  metamorphism,  where,  under  the  influence  of 
intense  shearing,  their  original  structure  has  been  more  or  less 
completely  effaced,  and  a  new  crystalline  rearrangement  has 
been  developed  in  them,  converting  them  into  schists. 


208  GEOLOGY 


CHAPTER  XIV. 

ERUPTIVE  ROCKS  AND  MINERAL  VEINS. 

NOT  only  have  sedimentary  formations  since  their  dep- 
osition been  hardened,  plicated,  fractured,  and  some- 
times even  turned  into  crystalline  schists,  but  into  the 
rents  opened  in  them  new  masses  of  mineral  matter  have  been 
introduced  which,  in  many  regions,  have  entirely  changed  the 
structure  of  the  crust  below  and  the  appearance  of  the  surface 
above.  Broadly  speaking,  there  are  two  ways  in  which  these 
new  masses  have  been  wedged  into  their  places.  First  of  all, 
eruptive  material  in  a  molten,  or  at  least  in  a  viscous  or  plastic 
condition,  has  been  thrust  upward  into  the  cool  and  consolidated 
crust  of  the  earth;  and  in  the  next  place,  various  ores  and 
minerals  have  been  deposited  from  solution  in  cracks  and  fissures, 
which  they  have  entirely  filled  up.  To  each  of  these  two  kinds 
of  later  rocks  attention  will  be  given  in  this  chapter. 

ERUPTIVE  ROCKS. 

The  rise  of  eruptive  matter  thrust  upwards  from  lower  depths 
within  the  planet  is  one  of  the  causes  by  which  the  structure 
of  the  crust  has  been  most  seriously  affected.  In  Chapter  IX 
reference  was  made  to  some  of  the  features  connected  with  the 
protrusion  of  molten  rocks  in  the  production  of  volcanoes,  and 
more  particularly  to  those  subterranean  changes  which,  when 
all  the  outer  and  ordinary  tokens  of  a  volcano  have  been  swept 
away,  remain  as  evidence  of  former  volcanic  action,  even  in  dis- 
tricts where  every  symptom  of  volcanic  activity  has  long  vanished. 
We  have  now  to  inquire,  generally,  in  what  forms  eruptive  matter 
has  been  built  into  the  earth's  crust,  and  what  changes  it  has 
produced  there,  apart  from  those  superficial  manifestations  which 
are  the  visible  signs  of  volcanic  action. 

When  a  mass  of  lava  is  forced  upwards  from  the  heated  in- 


ERUPTIVE  ROCKS  AND  MINERAL  VEINS  209 

terior  of  the  earth  towards  the  surface  the  form  which  it  finally 
takes  and  in  which  it  cools  and  solidifies  must  depend  upon  the 
shape  of  the  rent  or  cavity  into  which  it  has  been  thrust.  We 
may  compare  such  a  mass  to  a  quantity  of  melted  iron  escaping 
from  a  blast-furnace.  The  shape  taken  by  the  iron  will,  of 
course,  be  fixed  by  that  of  the  mould  into  which  it  is  allowed 
to  run.  The  crust  of  the  earth,  as  was  pointed  out  in  the 
previous  chapter,  has  undergone  extensive  movements,  whereby 
its  rocks  have  been  crumpled  and  broken.  It  consequently  pre- 
sents in  different  parts  very  various  degrees  of  resistance  to  any 
force  acting  upon  it  from  below.  The  eruptive  materials  have 
sometimes  risen  in  the  fissures,  sometimes  have  forced  their  way 
between  the  beds  and  joints  of  the  strata.  According  to  the 
form  of  the  mould  in  which  they  have  solidified,  we  may  classify 
the  eruptive  rocks  of  the  crust  into  (1)  bosses;  (2)  sheets; 
(3)  veins  and  dykes;  and  (4)  necks. 

BOSSES. — These  are  circular,  elliptical,  or  irregularly  shaped 
masses  of  rock  which,  while  still  in  a  liquid  or  viscous  state, 
have  been  ejected  into  irregular  rents  of  the  earth's  crust  and 
have  solidified  there.  They  consist  of  various  crystalline  rocks, 
more  especially  granite,  syenite,  quartz-porphyry,  diorite,  diabase, 
and  basalt-rocks,  and  vary  in  width  from  a  few  yards  to  several 
miles.  Being  generally  harder  than  the  surrounding  rocks,  they 
commonly  stand  up  as  prominent  knobs,  hills,  or  ridges.  Their 
presence  at  the  surface,  however,  is  due,  not  to  their  original 
protrusion  there,  as  in  a  volcanic  cone,  but  to  the  removal  of 
the  overlying  part  of  the  original  crust  under  which  they  cooled 
and  consolidated.  Every  boss  is  thus  a  witness  of  the  extensive 
wearing  away  of  the  surface  of  the  land  (Fig.  105). 

In  some  large  bosses  there  may  have  been  a  complex  system 
of  fissures  in  which  the  eruptive  material  rose.  Forced  upwards 
into  these,  the  molten  rock  would  no  doubt  envelope  separated 
masses  of  the  crust,  and  might  bear  them  along  with  it  in  its 
ascent.  We  may  even  conceive  it  to  have  melted  down  such 
enveloped  masses.  Pushing  the  rocks  aside  and  thrusting  itself 
into  every  available  crack  in  them  the  eruptive  mass  would  work 
its  way  across  the  crust.  Where  it  succeeded  in  opening  a  pas- 
sage to  the  surface,  ordinary  volcanic  phenomena  would  take 
place,  such  as  disruption  of  the  ground,  ejection  of  stones  and 
ashes,  and  outflow  of  lava.  But,  no  doubt,  in  a  vast  number 


210  GEOLOGY 

of  cases,  no  such  communication  was  ever  effected.  The  eruptive 
material  paused  in  its  upward  passage  and  consolidated  below 
ground. 

No  rock  affords  more  interesting  bosses  than  granite.  Two 
features  are  especially  well  displayed  by  it  —  the  marginal  veins 
or  dykes  and  the  surrounding  ring  of  metamorphism  produced 
in  the  rocks  through  which  granite  has  risen.  Granite  has  in- 
vaded many  different  kinds  of  rocks,  and  has  effected  various 
kinds  of  change  in  them.  Bound  its  margin,  large  numbers 
of  veins  or  dykes  of  granite  or  quartz-porphyry  often  strike  out 
from  it  into  the  surrounding  rocks.  There  can  be  no  doubt 
that  these  are  portions  of  the  granite  material,  squeezed  into 
cracks  that  opened  in  the  crust  around  it  during  its  ascent. 
More  important  is  the  change  that  can  be  observed  to  have  taken 
place  in  the  rocks  immediately  surrounding  the  boss.  The 
granite  at  the  time  of  its  protrusion  was  probably  in  a  molten 
or  pasty  condition  and  impregnated  with  hot  water  or  steam 
and  vapours.  For  a  distance  varying  from  a  few  feet  up  to 
two  or  three  miles,  according  chiefly  to  the  size  of  the  granite 
mass,  the  rocks  next  to  it  have  undergone  alteration,  the  nature 
and  amount  of  which  appear  to  have  been  in  great  measure  de- 
pendent on  the  chemical  and  mineralogical  composition  of  the 
rocks  themselves.  This  metamorphism  consists  partly  in  mere 
induration,  but  still  more  in  the  development  of  new  minerals, 
or  a  new  crystalline  structure,  even  out  of  non-crystalline  sedi- 
mentary materials.  The  very  same  rock,  which  is  elsewhere 
a  dark  limestone  full  of  shells,  corals,  or  other  organic  remains, 
becomes  a  white  crystalline  marble  next  the  granite,  with  no 
trace  of  any  organisms,  and  so  unlike  its  usual  condition  that 
no  one  would  readily  believe  it  to  be  the  same  rock.  Again,  a 
dark  shaly  sandstone  or  graywacke  traced  towards  the  granite 
begins  to  show  an  increasing  amount  of  mica.  New  minerals 
likewise  make  their  appearance,  particularly  garnets,  until  the 
rock  entirely  loses  its  sedimentary  structure  and  becomes  a 
garnetiferous  mica-schist.  Shales  and  slates,  as  they  approach 
the  granite,  likewise  present  a  remarkable  development  of  fine 
mica-plates,  and  pass  into  argillaceous  schists  or  phyllites,  with 
crystals  of  chiastolite  or  other  minerals  developed  in  them.  The 
alteration  of  rocks  round  eruptive  masses  is  called  Contact- 
metamorphism. 


ERUPTIVE  ROCKS  AND  MINERAL  VEINS  211 

What  the  cause  may  be  of  this  remarkable  alteration  has  not 
yet  been  satisfactorily  made  out.  In  some  bosses,  the  mere  heat 
of  the  eruptive  material  was  probably  sufficient  to  produce  change. 
There  must  often  have  been  also  a  copious  discharge  of  hot 
vapours  and  water  which  would  powerfully  affect  the  adjacent 
rocks.  Silica  and  other  substances  might  then  be  introduced, 
leading  to  induration  and  new  chemical  rearrangements  of  the 
constituents.  The  protrusion  of  enormous  masses  of  granite 
may  also  have  given  rise  to  mechanical  movements  in  the  earth's 
crust,  like  those  which  have  produced  the  shearing  and  schistose 
structure,  seen  in  regional  metamorphism. 


a  * 

Fig.  105. —  Outline  and  section  of  a  Boss  (a)  traversing  stratified 
rocks    (6). 

SHEETS. —  Sometimes  the  easiest  passage  for  the  erupted  ma- 
terial from  below  has  lain  between  the  bedding  of  strata.  The 
molten  rock,  after  ascending  some  fissure  or  pipe  has  found 
its  farther  progress  barred,  and  has  escaped  by  forcing  up  the 
overlying  beds  and  thrusting  itself  in  below  them.  On  cooling 
and  consolidating,  it  appears  as  a  sheet  or  bed  intercalated 
between  older  rocks.  This  structure  is  represented  in  Fig.  107. 
Any  one  examining  such  a  section  on  the  ground,  might  naturally 
regard  the  sheet  s  as  a  bed  of  lava  erupted  at  the  surface  after 
the  formation  of  the  strata  a  and  before  that  of  &.  But  various 
features  characteristic  of  intrusive  or  subsequently  injected 
sheets  enable  us  to  distinguish  them  from  those  which  have  been 
poured  out  during  the  deposition  of  the  strata  among  which 
they  lie.  For  example,  intrusive  sheets  break  across  the  strata 
(as  at  d  in  Fig.  107)  and  send  veins  into  them.  They  are 


212 


GEOLOGY 


commonly  most  close-grained  along  their  edges,  where  they  have 
been  most  rapidly  chilled  by  contact  with  the  cool  rocks ;  while, 
on  the  contrary,  true  lava-streams  erupted  at  the  surface  are 
generally  most  slaggy  and  scoriform  on  their  upper  and  under 
surfaces.  Lastly,  they  have  generally  hardened  and  otherwise 
altered  the  rocks  above  and  below  them,  sometimes  baking  or 
even  fusing  them.  Where  these  characters  are  present,  we  may 
confidently  infer  that,  though  a  sheet  of  crystalline  rock,  so  far 
as  visible  at  the  surface,  may  seem  to  be  regularly  interstratified 
between  sedimentary  beds,  as  if  it  had  been  contemporaneously 
poured  forth  among  them,  it  has  nevertheless  been  thrust  in 
between  them  and  may  be  of  much  younger  date. 

On  the  other  hand,  a  truly  contemporaneous  sheet  or  group 
of  sheets  marking  the  actual  outpouring  of  lava-streams  at  the 


Fig.  106. —  Ground-plan  of  Granite-boss  with  ring  of  Contact-Metamor- 
phism ;  (a)  sandstones,  shales,  etc.,  dipping  at  high  angles  in  the 
direction  of  the  arrows;  (6)  zone  or  ring  within  which  these  rocks 
are  metamorphosed;  (c)  granite  sending  out  veins  into  &. 

surface,  during  the  deposition  of  the  strata  among  which  they 
now  lie,  may  be  recognised  by  equally  distinctive  characters. 
Thus  they  do  «not  break  across  nor  send  veins  into  the  overlying 
or  underlying  beds,  while  their  upper  and  under  surfaces  are 
usually  their  most  open  cellular  portions  though  they  are  often 
more  or  less  vesicular  or  amygdaloidal  throughout.  In  Fig.  108 
the  beds  marked  1,  2,  3,  and  4  are  sheets  of  different  lavas 
interstratified  contemporaneously  in  the  series  of  sandstones, 
shales,  limestones,  and  other  strata  among  which  they  lie.  Frag- 
ments of  them  are  not  infrequently  to  be  detected  in  the  overlying 
strata,  which  are  thus  shown  to  be  of  later  origin,  and  bands 
of  tuff  are  commonly  associated  with  them,  just  as  showers 
of  ashes  accompany  the  lava-streams  of  living  volcanoes.  As 


ERUPTIVE  ROCKS  AND  MINERAL  VEINS  213 

an  illustration  of  the  way  in  which  the  evidence  of  ancient  vol- 
canic action  may  be  gathered,  the  section  given  in  Fig.  109 
may  be  taken  supplementary  to  the  data  given  already  in  Chapter 
IX.  At  the  bottom  of  the  section  we  stand  on  the  slaggy  upper 
surface  of  a  lava-stream  (1)  which  was  poured  out  under  water, 
for  directly  above  it  comes  a  seam  of  dark  shale  (2)  representing 
fine  mud  that  was  deposited  from  suspension  in  water.  That 
volcanic  explosions  still  continued  after  the  outflow  of  the  lava, 
is  indicated  by  the  abundant  bits  of  slaggy  lava  and  volcanic 
detritus  scattered  through  the  shale,  and  that  the  scene  of  these 
operations  was  the  sea-floor  is  conclusively  proved  by  the  numer- 
ous shells,  crinoids,  and  other  marine  remains  that  lie  in  some 
bands  of  the  shale.  The  bottom  must  then  have  been  muddy 


a 

Fig.  107. —  Intrusive  Sheet. 

and  not  so  well  suited  as  it  afterwards  became  for  the  support 
of  life.  Above  the  shale  we  find  two  feet  of  limestone  (3) 
which  is  entirely  made  up  of  fragments  of  marine  organisms. 
These  creatures,  when  the  water  had  cleared,  continued  to  flourish 
abundantly  until  their  congregated  remains  formed  a  bed  of 
solid  limestone.  But  from  some  change  in  the  geography  of 
the  region,  currents  bearing  dark  mud  once  more  invaded  this 
part  of  the  sea  and  threw  down  the  material  that  now  forms 
the  band  of  shale  (4).  The  absence  of  organic  remains  in  this 
band  probably  shows  the  inroad  of  mud  to  have  destroyed  the 
life  that  had  previously  been  prolific.  When  this  condition  of 
things  had  been  brought  about,  renewed  volcanic  explosions  took 
place  in  the  neighbourhood.  First  came  showers  of  dust,  ashes, 
and  stones,  which  fell  over  the  sea,  and  are  now  represented  by 
the  band  of  tuff  (5).  Then  followed  the  outpouring  of  a  stream 
of  lava  (6),  with  its  characteristic  cellular  structure.  But  this 
did  not  quite  exhaust  the  vigour  of  the  volcano,  for  the  band  of 
tuff  (7)  points  to  successive  showers  of  dust  and  stones.  When 


214 


GEOLOGY 


the  explosion  ceased,  the  deposition  of  dark  mud,  which  had 
been  interrupted  by  the  volcanic  episode,  was  resumed,  and  the 
band  of  shale  (8)  was  laid  down.  From  the  fragments  of  ferns 
and  other  plants  in  this  shale  it  is  clear  that  land  was  not  far 
off.  The  sea  had  evidently  been  gradually  shallowing  by  the 


Fig.  108. —  Interstratified  or  contemporaneous  Sheets. 

infilling  of  sediment  and  volcanic  materials,  and  at  last,  on  the 
muddy  flat,  represented  by  the  layer  of  fire-clay  (9),  marshy 
vegetation  sprang  up  into  a  thick  jungle  like  the  mangrove- 
swamps  of  tropical  shores  at  the  present  day.  But  after  growing 
long  enough  to  form  the  bed  of  matted  vegetable  matter  now 
represented  by  the  coal-seam  (10),  the 
verdant  jungle  was  invaded  by  the  sea, 
and  sank  under  the  muddy  water  that 
threw  down  upon  its  submerged  surface 
the  gray  shale  (11).  In  this  shale  we 
detect  interesting  traces  of  the  renewal 
of  volcanic  activity,  more  especially  in 
occasional  large  blocks  of  lava,  which 
have  evidently  been  ejected  by  some 
volcanic  explosion,  as  in  the  example 
already  cited  in  Chap.  IX  (Fig.  38).  A 
more  vigorous  volcanic  outburst  poured 
out  the  stream  of  columnar  lava  (12) 
which  buried  the  whole  and  forms  the 
top  of  the  section. 

Tustrate"  evMe°ncet0  of        VEINS    AND   DYKES.— These  have  al- 

contemporaneous      vol-  ready  been  referred  to  in  Chapter  IX  as 
came  action.  •••.••  .  -,  -, 

part  of  the  evidence  for  volcanic  action. 

We  have  here  to  consider  how  they  occur  in  connection  with  the 
protrusion  of  eruptive  material  within  the  crust  of  the  earth. 
Where  the  material  so  erupted  has  solidified  in  a  vertical  or 


ERUPTIVE  ROCKS  AND  MINERAL  VEINS  215 

iiearly  vertical  fissure  so  as  to  form  a  wall-like  mass,  it  is  called 
a  dyke  (Fig.  43  and  d  in  Fig.  107).  Otherwise  the  portions 
of  erupted  rock  that  have  consolidated  in  irregular  rents  are 
known  as  veins. 

Veins  are  of  common  occurrence  round  bosses  of  granite,  where 
they  can  be  traced  into  the  parent  mass  from  which  they  have 
proceeded  (Fig.  106).  They  may  likewise  be  observed  in  con- 
nection with  intrusive  sheets  and  bosses  of  basalt  and  diorite, 
from  which  they  ramify  outwards  into  the  surrounding  rocks. 
Their  occurrence  there  is  one  of  the  proofs  of  the  intrusive 
character  and  subsequent  date  of  such  sheets. 

Dykes  vary  from  less  than  a  foot  to  70  feet  or  upward  in 
breadth,  and  run  in  nearly  straight  courses  sometimes  for  many 
miles.  They  consist  most  usually  of  diabase,  andesite,  basalt, 
or  allied  rock.  Sometimes  they  have  risen  along  lines  of  fault; 


Fig.  110. —  Map  of  Dykes  near  Muirkirk,  Ayrshire.  1.  Silurian  rocks. 
2.  Lower  Old  Red  Sandstone.  3.  Carboniferous  rocks.  /,  /,  /.  Faults. 
d,  d.  Dykes. 

but  in  hundreds  of  instances  in  Great  Britain,  they  do  not  appear 
to  be  connected  with  any  faults,  but  actually  cross  some  of 
the  largest  faults  in  the  country  without  being  deflected.  The 
remarkable  way  in  which  dykes  have  risen  through  a  complicated 
series  of  rocks  and  faults  and  have  preserved  their  courses  is 
exemplified  in  Fig.  110. 

Like  intrusive  sheets,  but  in  a  less  degree,  dykes  harden  or 
otherwise  alter  the  rocks  on  either  side  of  them;  they  likewise 
present  a  similar  closeness  of  grain  along  their  margins  where 
the  molten  rock  was  most  rapidly  chilled  by  coming  in  contact 


216 


GEOLOGY 


with  the  cold  walls  of  the  fissure.  Sometimes,  indeed,  their 
sides  are  coated  with  a  thin  crust  of  black  glass,  as  if  they 
had  been  painted  with  tar.  This  glass  represents  the  effect  of 
rapid  cooling  (see  Basalt-glass,  Chap.  XI).  No  doubt  the  whole 
rock  of  the  dyke,  at  the  time  when  it  rose  from  below  and  filled 
up  the  space  between  the  two  walls  of  its  opened  fissure,  was 
a  molten  glass.  The  portions  that  were  at  once  chilled  by  contact 
with  the  walls  adhered  as  a  layer  of  glass.  But  inside  this 
layer,  the  molten  rock  had  more  time  to  cool.  In  cooling,  its 
various  minerals  crystallised  and  the  present  crystalline  struc- 
ture was  developed.  But  even  yet,  though  most  of  the  rock  is 
formed  of  crystalline  minerals,  portions  of  the  original  glass 
may  not  infrequently  be  detected  between  them  when  thin  sec- 
tions are  placed  under  the  microscope. 


Fig.    111. —  Section  of  a   volcanic   neck.     The   dotted   lines   suggest   the 
original  form  of  the  volcano. 

NECKS. —  These  are  the  filled-up  pipes  or  funnels  of  former 
volcanic  vents.  Their  connection  with  volcanic  action  has  been 
already  alluded  to  in  Chap.  IX.  They  are  circular  or  elliptical  in 
ground-plan,  and  vary  in  diameter  from  a  few  yards  up.  to  a 
mile  or  more  (see  Figs.  40,  41,  42).  They  consist  of  some  form 
of  lava  (quartz-porphyry,  basalt,  diorite,  etc.)  or  of  the  frag- 
mentary materials  that,  after  being  ejected  from  the  volcanic 
chimney,  fell  back  into  it  and  consolidated  there.  They  occur 
more  particularly  in  districts  where  beds  of  lava  and  tuff  are 
interstratified  with  other  rocks.  The  necks  represent  the  vents 
from  which  these  volcanic  materials  were  ejected.  In  Fig.  Ill, 
for  example,  the  bed  of  lava  and  tuff  (b  b)  interstratified  be- 
tween the  strata  a  a  and  c  c  have  been  folded  into  an  anticline. 
In  the  centre  of  the  arch  rises  the  neck  (n)  which  has  probably 
been  the  chimney  that  supplied  these  volcanic  sheets,  and  which 


ERUPTIVE  ROCKS  AND  MINERAL  VEINS 


217 


has  been  filled  up  with  coarse  tuff,  and  traversed  with  dykes 
and  veins  of  basalt  (*).  The  dotted  lines,  suggestive  of  the 
outline  of  the  original  volcano,  may  serve  to  indicate  the  con- 
nection between  the  neck  and  its  volcanic  sheets,  and  also  the 
effects  of  denudation. 

Necks  are  frequently  traversed  by  dykes  (*  in  Fig.  Ill),  as 
we  know  also  to  be  the  case  with  the  craters  of  modern  volcanoes. 
The  rocks  surrounding  a  neck  are  sometimes  bent  down  round 
it,  as  if  they  had  been  dragged  down  by  the  subsidence  of  the 
material  filling  up  the  vent;  they  are  also  frequently  much 
hardened  and  baked.  When  we  reflect  upon  the  great  heat  of 
molten  lava  and  of  the  escaping  gases  and  vapours,  we  may  well 
expect  the  walls  of  a  volcanic  vent  to  bear  witness  to  the  effects 
of  this  heat.  Sandstones,  for  instance,  have  been  indurated  into 
quartzite,  and  shales  have  been  baked  into  a  hardened  clay  or 
porcelain-like  substance. 


a    a  3  4) 


Fig.  112. —  Section  of  a  Mineral  vein. 
MINERAL  VEINS. 

Into  the  fissures  opened  in  the  earth's  crust  there  have  been 
introduced  various  simple  minerals  and  ores  which,  solidifying 
there,  have  taken  the  form  of  Mineral  Veins.  These  materials 
are  to  be  distinguished  from  the  eruptive  veins  and  dykes  above 
described.  A  true  mineral  vein  consists  of  one  or  more  minerals 
filling  up  a  fissure  which  may  be  vertical,  but  is  usually  more  or 
less  inclined,  and  may  vary  in  width  from  less  than  an  inch 
up  to  150  feet  or  more.  The  commonest  minerals  (or  veinstones) 
found  in  these  veins  are  quartz,  calcite,  barytes,  and  fluor-spar. 
The  metalliferous  portions  (or  ores)  are  sometimes  native  metals 


218  GEOLOGY 

(gold  and  copper,  for  example),  but  are  more  usually  metallic 
oxides,  silicates,  carbonates,  sulphides,  chlorides,  or  other  com- 
binations. These  materials  are  commonly  arranged  in  parallel 
layers,  and  it  may  often  be  noticed  that  they  have  been  deposited 
in  duplicate  on  each  side  of  a  vein.  In  Fig.  112,  for  instance, 
we  see  that  each  wall  (w  w)  is  coated  with  a  band  of  quartz 
(1,  1),  followed  successively  by  one  of  blende  (sulphide  of  zinc, 
2,  2),  galena  (sulphide  of  lead,  3,  3),  barytes  (4,  4)  and  quartz 
(5,  5).  The  central  portion  of  the  vein  (6)  is  sometimes  empty 
or  may  be  filled  up  with  some  veinstone  or  ore.  Eemarkable 
variations  in  breadth  characterise  most  mineral  veins.  Some- 
times the  two  walls  come  together  and  thereafter  retire  from 
each  other  far  enough  to  allow  a  thick  mass  of  mineral  matter 
to  have  been  deposited  between  them.  Great  differences  may 
also  be  observed  in  the  breadth  of  the  several  bands  composing 
a  vein.  One  of  these  bands  may  swell  out  so  as  to  occupy  the 
whole  breadth  of  the  vein,  and  then  rapidly  dwindle  down.  The 
ores  are  more  especially  liable  to  such  variations.  A  solid  mass 
of  ore  may  be  found  many  feet  in  breadth  and  of  great  value; 
but  when  followed  along  the  course  of  the  vein,  may  die  away 
into  mere  strings  or  threads  through  the  veinstones. 

The  duplication  of  the  layers  in  mineral  veins  shows  that  the 
deposition  proceeded  from  the  walls  inwards  to  the  centre.  In 
the  diagram  (Fig.  112)  it  is  evident  that  the  walls  were  first 
coated  with  quartz.  The  next  substance  introduced  into  the 
vein  was  sulphide  of  zinc,  a  layer  of  which  was  deposited  on 
the  quartz.  Then  came  sulphide  of  lead,  and  lastly,  quartz  again. 
The  way  in  which  the  quartz-crystals  project  from  the  two  sides 
shows  that  the  space  between  them  was  free,  and,  as  above 
stated,  it  has  sometimes  remained  unfilled  up. 

There  appears  to  be  now  no  reason  to  doubt  that  the  sub- 
stances deposited  in  mineral  veins  were  mainly  introduced  dis- 
solved in  water.  Not  improbably  heated  waters  rose  in  the 
fissures,  and  as  they  cooled  in  their  ascent,  they  coated  the  walls 
with  the  minerals  which  they  held  in  solution.  These  minerals 
may  have  been  abstracted  from  the  surrounding  rocks  by  the 
permeating  water ;  or  they  may  have  been  carried  up  from  some 
deeper  source  within  the  crust.  During  the  process  of  infilling, 
or  after  it  was  completed,  a  fissure  has  sometimes  reopened,  and 
a  new  deposition  of  veinstones  or  ores  has  taken  place.  Now 


ERUPTIVE  ROCKS  AND  MINERAL  VEINS  219 

and  then,  too>  land-shells  and  pebbles  are  found  far  down  in 
mineral  veins,  showing  that  during  the  time  when  the  layers  of 
mineral  matter  were  being  deposited,  the  fissures  sometimes  com- 
municated with  the  surface. 

SUMMARY. —  In  this  chapter  it  has  been  shown  that,  in  many 
cases,  the  rents  in  the  earth's  crust  have  been  filled  up  with 
mineral  matter  introduced  into  them,  either  (i)  in  the  molten 
state,  or  (ii)  in  solution  in  water,  (i)  The  forms  assumed  by 
the  masses  of  eruptive  rock  injected  into  the  crust  of  the  earth 
have  depended  upon  the  shape  of  the  fissures  into  which  the 
melted  matter  has  been  poured,  as  the  form  of  a  cast-iron  bar 
is  regulated  by  that  of  the  mould  into  which  the  melted  metal  is 
allowed  to  run.  Taking  this  principle  of  arrangement,  we  find 
that  eruptive  rocks  may  be  grouped  into  (1)  Bosses,  or  irregu- 
larly-shaped masses,  which  have  risen  through  fissures  or  orifices, 
and  now,  owing  to  the  removal  of  the  rock  under  which  they 
solidified,  form  hills  or  ridges.  The  eruptive  material  sends 
out  veins  into  the  surrounding  rocks  which  are  sometimes  con- 
siderably altered,  forming  a  metamorphic  ring  round  the  eruptive 
rock.  (2)  Sheets  or  masses  which  have  been  thrust  between 
the  bedding-planes  of  strata.  These  resemble  truly  interstratified 
beds,  but  the  difference  between  the  two  kinds  of  structure  can 
be  readily  appreciated.  Interstratified  beds  mark  the  occurrence 
of  volcanic  phenomena  at  the  surface  during  the  time  of  the 
formation  of  the  strata  among  which  they  occur.  Intrusive 
sheets,  on  the  other  hand,  are  always  subsequent  in  date  to  the 
rocks  between  which  they  lie.  (3)  Veins  and  dykes,  consisting 
of  eruptive  rock  which  has  been  thrust  between  the  walls  of 
irregular  rents  or  straight  fissures.  (4)  Necks,  or  the  filled-up 
pipes  of  former  volcanic  vents,  (ii)  Mineral  veins  are  masses 
of  mineral  matter  which  has  been  deposited,  probably  from 
aqueous  solution,  between  the  walls  of  fissures  in  the  earth's 
crust,  and  consists  of  bands  of  veinstones  (quartz,  calcite,  barytes, 
etc.)  and  ores  (native  metals,  or  oxides,  sulphides,  etc.,  of 
metals) . 


220  GEOLOGY 


CHAPTER  XV. 

FOSSIL    REMAINS. 

IN  AN"  earlier  part  of  this  volume  (Chapter  VIII)  attention 
was  called  to  the  various  circumstances  under  which  the 
remains  of  plants  and  animals  may  be  entombed  and  pre- 
served in  sedimentary  accumulations.  When  these  remains  have 
thus  been  buried  they  are  known  as  Fossils. 

NATURE  AND  USE  OF  FOSSILS. —  The  word  "  fossil,"  meaning 
literally  "  dug  up,"  was  originally  applied  to  all  kinds  of  mineral 
substances  taken  out  of  the  earth ;  but  it  is  now  exclusively  used 
for  the  remains  or  traces  of  plants  and  animals  imbedded  by 
natural  causes  in  any  kind  of  rock,  whether  loose  and  incoherent, 
like  blown  sand,  or  solid,  like  the  most  compact  limestone.  It 
includes  not  only  the  actual  remains  of  the  organisms.  The 
empty  mould  of  a  shell  which  has  decayed  out  of  the  stone  that 
once  enveloped  it,  or  the  cast  of  the  shell  which  has  been  entirely 
replaced  by  inorganic  sand,  mud,  calcite,  silica,  etc.,  are  fossils. 
The  very  impressions  left  by  organisms,  such  as  the  burrow  or 
trail  of  a  worm  in  hardened  mud,  and  the  footprints  of  birds 
and  quadrupeds  upon  what  is  now  sandstone,  are  undoubted 
fossils.  In  short,  under  this  general  term  is  included  whatever 
bears  traces  of  the  form,  structure,  or  presence  of  organisms 
preserved  in  the  sedimentary  accumulations  of  the  surface,  or 
in  the  rocks  underneath. 

In  geological  history  fossils  are  of  fundamental  importance. 
They  enable  us  to  investigate  conditions  of  geography,  of  climate, 
and  of  life  in  ancient  times,  when  these  conditions  were  very- 
different  from  those  which  now  prevail  on  the  earth's  surface. 
They  likewise  furnish  the  ground  on  which  the  several  epochs 
of  geological  history  can  be  determined,  and  on  which  the  stages 
of  that  history  in  one  country  can  be  compared  with  those  in 
another.  So  valuable  and  varied  is  the  evidence  supplied  by 


FOSSIL  REMAINS  221 

fossils  to  the  geologist,  that  he  regards  them  as  among  the  most 
precious  documents  accessible  to  him  for  unravelling  the  past 
history  of  the  earth.  Some  knowledge  of  the  structure  and 
classification  of  plants  and  animals  is  essential  for  an  intelligent 
appreciation  of  the  use  of  fossils  in  geological  inquiry.  An 
effort  is  made  in  this  work  to  give  the  learner  such  aid  as  is  fairly 
within  its  scope  to  arrive  at  an  intelligent  understanding ;  but  it 
must  be  understood  that  for  adequate  information  on  this  sub- 
ject recourse  should  be  had  to  text-books  of  Botany  and  Zoology. 

CONDITIONS  FOR  THE  PRESERVATION  OF  ORGANIC  EEMAINS. — 
It  is  obvious  that  all  kinds  of  plants  and  animals  have  not  the 
same  chances  of  being  preserved  as  fossils.  In  the  first  place, 
only  those,  as  a  rule,  are  likely  to  become  fossils  whose  remains 
can  be  kept  from  decay  and  dissolution  by  being  entombed  in 
some  kind  of  deposit.  Hence  land-animals  and  plants  have, 
on  the  whole,  less  chance  of  preservation  than  those  living  in 
the  sea,  because  deposits  capable  of  receiving  and  securing  their 
remains  are  exceptional  on  land,  but  are  generally  distributed 
over  the  floor  of  the  sea.  We  should  expect,  therefore,  that 
among  the  records  of  past  time,  traces  of  marine  should  largely 
preponderate  over  traces  of  terrestrial  life.  Now  this  is  every- 
where the  case.  We  know  relatively  little  of  the  assemblages 
of  plants  and  animals  which  in  successive  epochs  have  lived  upon 
the  dry  land,  but  we  have  a  comparatively  large  amount  of  infor- 
mation regarding  those  which  have  tenanted  the  sea.  For  this 
reason,  marine  fossils  are  more  valuable  than  terrestrial,  in 
comparing  the  records  of  the  successive  epochs  of  geological 
history  in  different  parts  of  the  globe. 

In  the  second  place,  from  their  own  chemical  composition  and 
structure,  plants  and  animals  present  extraordinary  differences 
in  their  aptitude  for  preservation  as  fossils.  Where  they  possess 
no  hard  parts,  and  are  liable  to  speedy  decay,  we  can  hardly 
expect  that  they  should  leave  behind  them  any  enduring  relic 
of  their  existence.  Hence  a  large  proportion,  both  of  the  vege- 
table and  animal  kingdoms,  may  at  once  be  excluded  as  in- 
herently unlikely  to  occur  in  the  fossil  condition.  Of  course, 
under  exceptional  circumstances,  traces  of  almost  any  organism 
may  be  preserved,  and  therefore  we  should  probably  not  be 
justified  in  saying  that  by  no  chance  might  some  recognisable 


222  GEOLOGY 

vestige  of  it  be  found  fossil.  Nothing  seems  more  perishable 
than  the  tiny  gnats  and  other  forms  of  insect  life  that  fill  the 
air  on  a  summer  evening.  Yet  many  of  these  short-lived  flies 
have  been  sealed  up  within  the  resin  of  trees  (amber),  and  their 
structure  has  been  admirably  preserved.  Such  exceptional  in- 
stances, however,  only  bring  out  more  distinctly  how  large  a 
proportion  of  the  living  tribes  of  the  land  must  utterly  perish, 
and  leave  no  recognisable  record  of  their  ever  having  existed. 

But,  where  there  are  hard  parts  in  an  organism,  and  especially 
where,  from  their  chemical  composition,  they  can  for  some  time 
resist  decay,  they  may,  under  favourable  conditions,  be  buried 
in  sedimentary  deposits,  and  may  remain  for  indefinite  ages 
locked  up  there.  It  is  obvious,  therefore,  that  animals  possessing 
hard  parts  are  much  the  most  likely  to  leave  permanent  relics 
of  their  presence,  and  ought  to  occur  most  frequently  as  fossils. 
It  is  these  animals  whose  remains  are  preserved  in  peat-mosses, 
river-gravels,  lake-marls,  and  on  the  sea-floor  at  the  present  time. 
Yet,  if  we  were  to  judge  of  the  extent  of  the  whole  existing 
animal  kingdom  solely  from  the  fragmentary  remains  so  pre- 
served, what  an  utterly  inadequate  conception  of  it  we  should 
form !  So,  too>  if  we  estimate  the  variety  of  the  living  creatures 
of  past  time  merely  from  the  evidence  of  the  fossils  that  have 
chanced  to  be  preserved  among  the  rocks,  we  shall  probably 
arrive  at  quite  as  erroneous  a  conclusion.  There  can  be  no 
doubt  that  from  the  earliest  time  only  an  insignificant  fraction 
of  the  varied  life  of  each  period  has  been  preserved  in  the  fossil 
state,  as  is  unquestionably  the  case  at  the  present  day. 

DURABLE  PARTS  OF  PLANTS. —  The  essential  parts  of  the  solid 
framework  of  plants  consist  of  the  substances  known  as  cellulose 
and  vasculose,  which,  when  kept  in  dry  air,  or  when  waterlogged 
and  buried  in  stiff  mud,  may  remain  undecomposed  for  long 
periods.  The  timber  beams  in  the  roofs  and  floors  of  old  build- 
ings are  evidence  that,  under  favourable  conditions,  wood  may 
last  for  many  centuries.  Some  plants  eliminate  carbonate  of 
lime  from  solution  in  water,  and  form  with  it  a  solid  substance 
which  requires  no  further  treatment  to  enable  it  to  endure  for 
an  indefinite  period,  when  screened  from  the  action  of  water. 
Still  more  durable  are  the  remains  of  those  plants  which  abstract 
silica  and  build  it  up  into  their  framework,  such  as  the  diatoms 
of  which  the  frustules  become  remarkably  permanent  fossils,  in 


FOSSIL  REMAINS  223 

the  form  of  diatom-earth  or  tripoli-powder,  which  is  made  up 
of  them. 

DURABLE  PARTS  OF  ANIMALS. —  The  hard  parts  of  animals 
may  be  preserved  with  little  or  no  chemical  change,  and  remain 
as  durable  relics.  The  hard  horny  integuments  of  insects,  arach- 
nids, Crustacea,  and  some  other  animals,  are  composed  essentially 
of  the  substance  called  chitin,  which  can  long  resist  decomposi- 
tion, and  which  may  therefore  be  looked  for  in  the  sedimentary 
deposits  of  the  present  time,  as  well  as  of  former  periods.  The 
chitin  of  some  fossil  scorpions,  admirably  preserved  among  the 
Carboniferous  rocks  of  Scotland,  can  hardly  be  distinguished 
from  that  of  the  living  scorpion.  Many  of  the  lower  forms  of 
animal  life  secrete  silica,  and  their  hard  parts  are  consequently 
easily  preserved,  as  in  the  case  of  radiolaria  and  sponges.  In  the 
great  majority  of  instances,  however,  the  hard  parts  of  inverte- 
brates consist  mainly  of  carbonate  of  lime,  and  are  readily  pre- 
served among  sedimentary  deposits.  The  skeletons  of  corals,  the 
plates  of  echinoderms,  and  the  shells  of  molluscs,  are  examples 
of  the  abundance  of  calcareous  organisms,  and  the  frequency 
of  their  remains  in  the  fossil  state  shows  how  well  fitted  they 
are  for  preservation.  Among  vertebrates  the  hard  parts  consist 
chiefly  of  phosphate  of  lime.  In  some  forms  (ganoid  fishes  and 
crocodiles,  for  example)  this  substance  is  partly  disposed  outside 
the  body  (exo-skeleton)  in  the  form  of  scales,  scutes,  or  bony 
plates.  But  more  usually  it  is  confined  to  the  internal  skeleton 
(endo-skeleton).  It  is  mainly  by  their  bones  and  teeth  that 
the  higher  vertebrates  can  be  recognised  in  the  fossil  state. 
Sometimes  the  excrement  has  been  preserved  (Coprolites),  and 
may  furnish  information  regarding  the  food  of  the  animals, 
portions  of  undigested  scales,  teeth,  and  bones  being  traceable 
in  it  (Fig.  65). 

FOSSILISATION, —  The  process  by  which  the  remains  of  a  plant 
or  animal  are  preserved  in  the  fossil  state  is  termed  Fossilisation. 
It  varies  greatly  in  details,  but  all  these  may  be  reduced  to  three 
leading  types. 

1.  Entire  or  partial  preservation  of  the  original  substance. — 
In  rare  instances,  the  entire  animal  or  plant  has  been  preserved, 
of  which  the  most  remarkable  examples  are  those  where  carcases 
of  the  extinct  mammoth  have  been  sealed  up  in  the  frozen  mud 
and  peat  of  Siberia,  and  have  thus  been  preserved  in  ice.  Insects 


224 


GEOLOGY 


have  been  involved  in  the  resin  of  trees,  and  may  now  be  seen, 
embalmed  like  mummies,  in  amber.  More  usually,  however,  a 
variable  proportion  of  the  organic  matter  has  passed  away,  and 
its  more  durable  parts  have  been  left,  as  in  the  carbonisation 
of  plants  (peat,  lignite,  coal)  and  the  disappearance  of  the 
organic  matter  from  shells  and  bones,  which  then  become  dry 
and  brittle  and  adhere  to  the  tongue. 

2.  Entire  removal  of  the  original  substance  and  internal  struc- 
ture,, only  the  external  form  being  preserved. —  When  a  dead 
animal  or  plant  has  been  entombed,  the  mineral  matter  in  which 
it  lies  hardens  round  it  and  takes  a  mould  of  its  form.  This 
may  be  accomplished  with  great  perfection  if  the  material  is 
sufficiently  fine-grained  and  solidifies  before  the  object  within 
has  time  to  decay.  Carbonate  of  lime  and  silica  are  especially 
well  adapted  for  taking  the  moulds  of  organisms,  but  fine  mud, 


Fig.  113. —  Common  Cockle  (Cardium  edule)  ;  (a)  side  view  of  both 
valves;  (&)  mould  of  the  external  form  of  one  valve  taken  in  plaster 
of  Paris;  (c)  side  view  of  cast  in  plaster  of  Paris  of  interior  of  the 
united  valves. 

marl,  and  sand,  are  also  effective.  The  organism  may  then 
entirely  decay,  and  its  substance  may  be  gradually  removed  by 
percolating  water,  leaving  a  hollow  empty  space  or  mould  of  its 
form.  Such  moulds  are  of  frequent  occurrence  among  fossil- 
iferous  rocks,  and  are  specially  characteristic  of  molluscs,  the 
shells  of  which  are  so  abundant,  and  occur  imbedded  in  so  many 
different  kinds  of  material.  Sometimes  it  is  the  external  form 
of  the  shell  that  has  been  taken,  the  shell  itself  having  entirely 
disappeared;  in  other  cases  a  cast  of  the  interior  of  the  shell 
has  been  preserved.  How  different  these  two  representations 
of  the  same  shell  may  be  is  shown  in  Fig.  113,  wherein  a  repre- 
sents a  side  view  of  the  common  cockle,  while  c  is  a  cast  of  the 


FOSSIL  REMAINS  225 

interior  of  the  shell  in  plaster  of  Paris.  The  contrast  between 
a  mould  of  the  outside  and  inside  of  the  same  shell  is  shown  by 
the  difference  between  &  and  c,  which  are  both  impressions  taken 
in  plaster. 

After  the  decay  and  removal  of  the  substance  of  the  enclosed 
organisms,  the  moulds  may  be  filled  up  with  mineral  matter, 
which  is  sometimes  different  from  the  surrounding  rock.  The 
empty  cavities  have  formed  convenient  receptacles  for  any  deposit 
which  permeating  water  might  introduce.  Hence  we  find  casts 
of  organisms  in  sand,  clay,  ironstone,  silica,  limestone,  pyrites, 
and  other  mineral  substances.  Of  course,  in  such  cases,  though 
the  external  form  of  the  original  organism  is  preserved,  there 
is  no  trace  of  internal  structure.  No  single  particle  of  the  cast 
may  ever  have  formed  part  of  the  plant  or  animal. 

3.  Partial  or  entire  petrifaction  of  organic  structure  by  molec- 
ular replacement. —  Plants  and  animals  which  have  undergone 
this  change  have  had  their  substance  gradually  removed  and 
replaced,  particle  by  particle,  with  the  mineral  matter.  This 
transformation  has  been  effected  by  percolating  water  contain- 
ing mineral  solutions,  and  has  proceeded  so  tranquilly,  that 
sometimes  not  a  delicate  tissue  in  the  internal  structure  of  a 
plant  has  been  displaced,  and  yet  so  rapidly,  that  the  plant  had 
not  time  to  rot  before  the  conversion  was  completed.  Accordingly, 
in  true  petrifactions,  that  is,  plants  or  animals  of  which  the 
structure  has  been  more  or  less  perfectly  preserved  in  stone,  the 
petrifying  material  is  always  such  as  may  have  been  deposited 
from  water.  The  most  common  substance  employed  by  nature 
in  the  process  of  petrification  is  carbonate  of  lime,  which,  as 
we  have  seen,  is  almost  always  present  in  the  water  of  springs 
and  rivers.  Organic  structures  replaced  by  this  substance  are 
said  to  be  calcified.  Frequently  the  carbonate  of  lime  has  as- 
sumed, more  or  less  completely,  a  crystalline  structure  after  its 
deposition,  and  in  so  doing  has  generally  injured  or  destroyed 
the  organic  structure  which  it  originally  replaced.  Where  the 
calcareous  matter  of  an  organism  has  been  removed  by  percolat- 
ing water,  as  often  happens  in  sands,  gravels,  or  other  porous 
deposits,  the  fossil  is  said  to  be  decalcified.  Another  abundant 
petrifying  medium  in  nature  is  silica,  which,  in  its  soluble  form, 
is  generally  diffused  in  terrestrial  waters,  where  humous  acids 
or  organic  matter  are  present  in  solution.  The  replacement  of  or- 


226  GEOLOGY 

game  structures  by  silica,  called  silicification,  furnishes  the  most 
perfect  form  of  petrifaction.  The  interchange  of  mineral  mat- 
ter has  been  so  complete  that  even  the  finest  microscopic 
structures  have  been  faithfully  preserved.  Silicified  wood  is  an 
excellent  example  of  this  perfect  replacement.  Sulphides,  which 
are  often  produced  by  the  reducing  action  of  decaying  organic 
matter  upon  sulphates,  occur  also  as  petrifying  media,  the  most 
common  being  the  iron  sulphide,  usually  in  the  less  stable  form 
a  marcasite,  but  sometimes  as  pyrite.  Carbonate  of  iron  like- 
wise frequently  replaces  organic  structures;  the  clay-ironstones 
of  the  Carboniferous  system  abound  with  the  remains  of  plants, 
shells,  fishes,  and  other  organisms  which  have  been  converted 
into  siderite  (Fig.  62). 

The  chief  value  of  fossils  in  geology  is  to  be  found  in  the 
light  which  they  cast  upon  former  conditions  of  geography  and 
climate,  and  in  the  clue  which  they  furnish  to  the  relative  ages 
of  different  geological  formations. 

1.   HOW  FOSSILS  INDICATE  FORMER  CHANGES  IN   GEOGRAPHY. 

—  Terrestrial  plants  and  animals  obviously  point  to  the  existence 
of  land.  If  their  remains  are  found  in  strata  wherein  most  of 
the  fossils  are  marine,  they  usually  show  that  the  deposits  were 
laid  down  upon  the  sea-floor  not  far  from  land.  But  where 
they  occur  in  the  positions  in  which  they  lived  and  died,  they 
prove  that  their  site  was  formerly  a  land-surface.  The  stumps 
of  trees  remaining  in  their  positions  of  growth,  with  their  roots 
branching  out  freely  from  them  in  the  clay  or  loam  underneath, 
undoubtedly  mark  the  position  of  an  ancient  woodland.  If,  with 
these  remains,  there  are  associated  in  the  same  strata  wing-cases 
of  beetles,  bones  of  birds  and  of  land-animals,  additional  cor- 
roborative evidence  is  thereby  obtained  as  to  the  existence  of 
the  ancient  land.  More  usually,  however,  it  is  by  deposits  left 
on  lake-bottoms  that  the  land  of  former  periods  of  geological 
history  is  known.  As  already  pointed  out  (Chapter  IV),  the 
fine  mud  and  marl  of  lakes  receive  and  preserve  abundant  relics 
of  the  vegetation  and  animal  life  of  the  surrounding  regions. 
As  illustrations  of  lacustrine  formations,  from  which  most  of 
our  knowledge  of  the  contemporary  terrestrial  life  is  obtained, 
reference  may  be  made  to  the  Molasse  of  Switzerland,  the  lime- 
stones and  marls  of  the  Limagne  d'Auvergne,  and  the  vast  depth 
of  strata  from  which  so  rich  an  assemblage  of  plant  and  animal 


FOSSIL  REMAINS  227 

remains  has  been  obtained  in  the  Western  Territories  of  the 
United  States  (see  Chapter  XXV).  Alternations  of  buried 
forests  or  peat-mosses,  with  lake  deposits,  show  how  lakes  have 
successively  increased  and  diminished  in  volume.  The  frequent 
occurrence  of  a  bed  of  lacustrine  marl  at  the  bottom  of  a  peat- 
bog proves  how  commonly  shallow  lakes  have  been  filled  up  and 
displace  by  the  growth  of  marshy  vegetation. 

Remains  of  marine  plants  and  animals  almost  invariably 
demonstrate  that  the  locality  in  which  they  are  found  was  once 
covered  by  the  sea.  Exceptions  to  this  rule  are  so  few  as  hardly 
to  be  worthy  of  special  notice,  as,  for  instance,  when  molluscs, 
crustaceans,  and  other  forms  of  marine  life  are  carried  up  by 
sea-birds  to  considerable  elevations,  where,  after  their  soft  parts 
have  been  eaten,  their  hard  shells  and  crusts  may  be  preserved 
in  truly  terrestrial  deposits,  or  when  sea-shells,  tossed  up  by 
breakers  above  the  tide-line,  are  swept  inland  by  wind. 

Rolled  fragments  of  shells,  mingled  in  well-rounded  gravel  and 
sand,  point  to  some  former  shore  where  these  materials  were 
ground  down  by  beach-waves.  Fine  muddy  sediment  containing 
unbroken  shells,  echinoderms,  crustaceans,  and  other  relics  of  the 
sea  indicate  deeper  water  beyond  the  scour  of  waves,  tides,  and 
currents.  Beds  of  limestone,  full  of  corals  and  crinoids,  mark  the 
site  of  a  clear  sea,  in  which  these  organisms  were  allowed  to 
flourish  undisturbed  for  many  generations.  It  may  often  be 
observed  that  the  fossils,  which  are  abundant  and  large  in  a 
limestone,  become  few  in  number  and  small  in  size  in  an  over- 
lying bed  of  shale  or  clay ;  or  that  they  wholly  disappear  in  the 
argillaceous  rock.  The  meaning  of  this  can  hardly  be  mistaken. 
The  clear  water  in  which  the  marine  creatures  were  able  to  build 
up  the  limestone  was  at  last  invaded  by  some  current  carrying 
mud.  Consequently,  while  the  more  delicate  forms  perished, 
others  continued  to  live  on  in  diminished  numbers  and  dwarfed 
development,  until  at  last  the  muddy  sediment  settled  down  so 
thickly  that  the  animals,  whose  hard  parts  might  have  been  pre- 
served, were  driven  away  from  that  area  of  the  sea-bottom. 

2.  How  FOSSILS  INDICATE  FORMER  CONDITIONS  OF  CLIMATE. — 
The  remains  of  plants  or  animals  characteristic  of  tropical  coun- 
tries may  be  taken  to  bear  witness  to  a  tropical  climate  at  the 
time  which  they  represent.  If  for  example,  a  deposit  were  found 
containing  leaves  of  palms  and  bones  of  tigers,  lions  and  ele- 


228  GEOLOGY 

phants,  we  should  infer  that  it  was  formed  in  some  tropical  coun- 
try, such  as  the  warmer  parts  of  Africa  or  Asia.  On  the  other 
hand,  were  a  stratum  to  yield  leaves  of  a  small  birch  and  willow, 
with  bones  of  reindeer,  musk-ox,  and  lemming,  we  would  regard 
it  as  evidence  of  a  cold  climate.  Such  inferences,  however,  must 
be  based  either  upon  the  occurrence  of  the  very  same  species  as 
are  now  living,  and  the  characteristic  climate  of  which  is  known, 
or  upon  assemblages  of  plants  or  animals  which  may  be  compared 
with  corresponding  assemblages  now  living.  We  may  be  toler- 
ably confident  that  the  existing  reindeer  has  always  been  restrict- 
ed to  a  cold  climate,  and  that  the  living  elephants  have  as  char- 
acteristically been  confined  to  warm  climates.  But  it  would  be 
rash  to  assume  that  all  deer  prefer  cold  and  all  elephants  choose 
heat.  The  bones  of  an  extinct  variety  of  elephant  and  one  of 
rhinoceros,  have  long  been  known  as  occurring  even  up  within 
the  Arctic  regions,  and  when  these  remains  were  first  found  the 
conclusion  was  naturally  drawn  that  they  proved  the  former  ex- 
istence of  a  warm  climate  in  the  far  north.  But  the  subse- 
quent discovery  of  entire  carcases  of  the  animals  covered  with 
a  thick  mat  of  woolly  hair,  showed  that  they  were  adapted  for 
life  in  a  cold  climate,  and  their  occurrence  in  association  with  the 
remains  of  animals  which  still  live  in  the  Arctic  regions,  proved 
beyond  doubt  that  the  original  inference  regarding  them  was 
erroneous.  In  drawing  conclusions  as  to  climate  from  fossil 
evidence,  it  is  always  desirable  to  base  them  upon  the  concurrent 
testimony  of  as  large  a  variety  of  organisms  as  possible,  and  to 
remember  that  they  become  less  and  less  reliable  in  proportion 
as  the  organisms  on  which  they  are  founded  depart  from  the 
species  now  living. 

3.  How  FOSSILS  INDICATE  GEOLOGICAL  CHRONOLOGY. —  As 
the  result  of  careful  observations  all  over  the  world,  it  has  been 
ascertained  that  in  the  youngest  strata  the  organic  remains  are 
nearly  or  quite  the  same  as  species  now  living,  but  that,  as  we 
proceed  into  older  strata,  the  number  of  existing  species  dimin- 
ishes, and  the  number  of  extinct  species  increases,  until  at  last 
no  living  species  is  to  be  found.  Moreover,  the  extinct  species 
found  in  younger  strata  disappear  as  we  trace  them  back  into 
older  rocks,  and  itieir  places  are  taken  by  other  extinct  species. 
Every  great  series  of  fossiliferous  rocks  is  thus  characterised  by 
its  own  peculiar  assemblage  of  species.  Not  only  do  the  species 


FOSSIL  REMAINS  229 

change ;  the  genera,  too,  disappear  one  by  one  as  we  follow  them 
into  older  rocks,  until  among  the  earliest  strata  only  a  few  of  the 
living  genera  are  represented.  Whole  families  and  orders  of  ani- 
mals which  once  flourished  have  utterly  vanished  from  the  living 
world,  and  we  only  know  of  their  existence  from  the  remains  of 
them  preserved  among  the  rocks. 

A  certain  definite  order  of  succession  has  been  observed  among 
the  organic  remains  imbedded  in  the  stratified  rocks  of  the  earth's 
crust,  and  this  order  has  been  found  to  be  broadly  alike  all  over 
the  world.  The  fossils  of  the  oldest  fossiliferous  rocks  of 
Europe,  for  instance,  are  like  those  of  the  oldest  fossiliferous 
rocks  of  Asia,  Africa,  America,  and  Australasia,  and  those  of  each 
succeeding  series  of  rocks  follow  the  same  general  sequence.  It 
is  obvious,  therefore,  that  fossils  supply  us  with  an  invaluable 
means  of  fixing  the  relative  position  of  rocks  in  the  series  of 
geological  formations.  Whether  or  not  the  same  type  of  fossils 
was  always  contemporaneous  over  the  whole  planet  cannot  be  de- 
termined ;  but  it  generally  occupied  the  same  place  in  the  proces- 
sion of  life.  Hence  stratified  formations,  which  may  be  quite 
unlike  each  other  in  regard  to  the  nature  of  their  component  ma- 
terials, if  they  contain  similar  organic  remains,  may  be  compared 
with  each  other,  and  classed  under  the  same  name. 

Fossils  Characteristic  of  particular  subdivisions  of  the  series  of 
geological  formations  are  known  as  type-fossils,  of  which  the  fol- 
lowing are  examples : — 

Lepidodendra  and  Sigillarise,  characteristic  of  Old  Red  Sandstone  and 
Carboniferous  rocks. 

Cycads,   characteristic  of  Mesozoic   rocks. 

Graptolites,    characteristic   of   Silurian    rocks. 

Trilobites  Cambrian  to  Carboniferous  rocks  (Figs. 

118,  125,  etc.). 

Cystideans,  characteristic  of  Silurian  rocks   (Fig.  123). 

Blastoids  Carboniferous  rocks   (Fig.  150). 

Hippurites  Cretaceous  rocks. 

Orthoceratites         "  Palaeozoic   rocks    (Figs.    130,    137.) 

Ammonites  Mesozoic  rocks   (Figs.  167,  176,  190). 

Cephalaspid  fishes  "  Silurian,  Old  Red  Sandstone. 

Ichthyosaurus  and  Plesiosaurus  —  Mesozoic  rocks   (Fig.   180). 

Iguanodon  —  Cretaceous  rocks   (Fig.   192). 

Toothed  birds  —  Cretaceous  rocks. 

Nummulites,  Palseotherium,  Anoplotherium,  Deinocerata,  character- 
istic of  older  Tertiary  rocks. 

Mastodon,  Elephas,  Equus,  Cervus,  Hyaena,  Apes,  characteristic  of 
younger  Tertiary  and  Recent  rocks. 


230  .  GEOLOGY 

By  attentive  study  and  comparison,  the  fossiliferous  rocks  in 
different  countries  have  been  subdivided  into  sections,  each 
characterised  by  its  own  f  acies  or  type  of  organic  remains.  Con- 
sequently, beginning  with  the  oldest  and  proceeding  upward  to 
the  youngest,  we  advance  through  natural  chronicles  of  the  suc- 
cessive tribes  of  plants  and  animals  which  have  lived  on  the 
earth's  surface.  These  chronicles,  consisting  of  sandstones, 
shales,  limestones,  and  the  other  kinds  of  stratified  deposits, 
form  what  is  called  the  Geological  Record.  In  order  to  establish 
their  true  sequence  in  time,  their  Order  of  Superposition  must 
first  be  determined;  that  is,  it  is  requisite  to  know  which  lie  at 
the  bottom,  and  must  have  been  formed  first,  and  in  what  order 
the  others  succeed  them.  When  this  fundamental  question  has 
once  been  settled,  then  the  fossils  characteristic  of  each  group  of 
strata  serve  as  a  guide  for  recognising  that  group  wherever  it  may 
be  found. 

While  fossils  enable  us  to  divide  the  Geological  Record  into 
chapters,  they  also  show  how  strikingly  imperfect  this  record  is 
as  a  history  of  the  plants  and  animals  that  have  lived  on  the 
surface  of  the  earth,  and  of  the  revolutions  which  that  surface 
has  undergone.  We  may  be  sure  that  the  progress  of  life,  from 
its  earliest  appearance  in  lowly  forms  of  plant  or  animal,  has  been 
continuous  up  to  the  present  condition  of  things.  But  in  the 
Geological  Record  there  occur  numerous  gaps.  The  fossils  of  one 
group  of  rocks  are  succeeded  by  a  more  or  less  completely  differ- 
ent series  in  the  next  group.  At  one  time  it  was  supposed  that 
such  breaks  in  the  continuity  of  the  record  marked  terrestrial 
convulsions  which  caused  the  destruction  of  the  plants  and  ani- 
mals of  the  time,  and  were  followed  by  the  creation  of  new  tribes 
of  living  things.  But  evidence  has  every  year  been  augmenting 
to  indicate  that  no  such  general  destruction  and  fresh  creation 
ever  took  place.  The  gaps  in  the  record  mark  no  real  interrup- 
tion of  the  life  of  the  globe.  They  are  rather  to  be  looked  upon 
as  chapters  that  have  been  torn  out  of  the  annals,  or  which  never 
were  written.  We  have  already  learnt  in  Chapter  VIII  how 
many  chances  there  must  be  against  the  preservation  of  any- 
thing like  a  complete  record  of  the  life  of  the  globe  at  any  par- 
ticular time.  It  is  also  clear  that  even  where  the  chronicle  may 
have  been  comparatively  full,  it  is  exposed  to  many  dangers  after- 
wards. The  rocks  containing  it  may  be  hidden  beneath  the  sea, 


FOSSIL  REMAINS 


231 


or  raised  up  into  land  and  entirely  worn  away,  or  entombed  be- 
neath volcanic  ejections,  or  so  crushed  and  crumpled  as  to  become 
no  longer  legible. 

Taking  fossils  as  a  guide,  geologists  have  partitioned  the  fos- 
siliferous  rocks  into  what  are  called  stratigraphical  subdivisions 
as  follows : —  A  bed,  or  limited  number  of  beds,  in  which  one  or 
more  distinctive  species  of  fossils  occur,  is  called  a  zone 
or  horizon,  and  may  be  named  after  its  most  typical  fossil.  Thus 
in  the  Lias,  the  zone  in  which  the  ammonite  known  as  Am- 
monites Jamesoni  occurs,  is  spoken  of  as  the  "  zone  of  Am- 
monites Jamesoni"  or  "Jamesoni-zone."  Two  or  more  zones, 
united  by  the  occurrence  in  them  of  a  number  of  the  same  char- 
acteristic species  or  genera,  form  what  are  known  as  Beds  or  an 
Assise.  Two  or  more  of  such  beds  or  assises  may  be  termed  a 
Group  or  Stage.  Where  the  number  of  assises  in  a  stage  is  large 
they  may  be  subdivided  into  Sub-stages  or  Sub-groups.  The 
stage  or  group  will  then  consist  of  several  sub-stages,  and  each 
sub-stage  or  sub-group  of  several  assises.  A  number  of  groups 
or  stages  is  combined  into  a  Series,  Section,  or  Formation,  and 
a  number  of  series,  sections,  or  formations  constitute  a  System. 
A  number  of  systems  are  connected  together  to  form  each  of  the 
great  divisions  of  the  Geological  Eecord.  This  classification  will- 
be  best  understood  if  placed  in  tabular  form,  as  in  the  subjoined 
subdivisions,  which  occur  in  the  Cretaceous  System.1 


Stratigraphical    com- 
ponents. 

A  stratum,  layer,  seam, 
or  bed,  or  a  number 
of  such  minor  subdi- 
visions, characterised 
by  some  distinctive 
fossil. 

Two  or  more  zones 

Two  or  more  sets  of 
connected  beds  or 
assises 


Descriptive  Names. 


=  Zone  or  horizon, 


Beds  or  an  assise. .. . 

I  Group  or  stage,  which 
I       may    be    subdivided 
I        into    sub-groups    or 
sub-stages 

Series,    section,    or   for- 
mation 


Two  or  more  groups  or 
stages 

Several    related    forma- 
tions 
1  For  an  account  of  the  Cretaceous  System,  see    Chapter  XXIV. 


System 


Examples  from  the  Cre- 
taceous   System. 


Zone  of  Pecten  asper. 


Warminster   beds. 

Cenomanian  stage,  com- 
prising the  Rotho- 
magian  and  Caren- 
tonian  sub-stages. 


Neocomian    formation. 
Cretaceous  System. 


232  GEOLOGY 

The  names  by  which  the  larger  subdivisions  of  the  Geological 
Record  are  known  have  been  adopted  at  various  times  and  on  no 
regular  system.  Some  of  them  are  purely  lithological ;  that  is, 
they  refer  to  the  mere  mineral  nature  of  the  strata,  apart  alto- 
gether from  their  fossils,  such  as  Coal-measures,  Chalk,  Green- 
sand,  Oolite.  These  names  belong  to  the  early  years  of  the  prog- 
ress of  geology,  before  the  nature  and  value  of  organic  remains 
had  been  definitely  realised.  Other  epithets  have  been  suggested 
by  localities  where  the  strata  occur,  as  London  Clay,  Oxford  Clay, 
Mountain  Limestone.  The  more  recent  names  for  the  larger 
divisions  have,  in  general,  been  chosen  from  districts  where  the 
strata  are  typically  developed,  or  where  they  were  first  critically 
studied,  e.g.  Silurian,  Devonian,  Permian,  Jurassic.  In  some 
cases,  the  larger  subdivisions  have  received  names  from  some  dis- 
tinguishing feature  in  their  fossil  contents,  as  Eocene,  Miocene, 
Pliocene.1  But  it  is  mainly  to  the  minor  sections  that  the  char- 
acters of  the  fossil  contents  have  supplied  names. 

The  designation  of  any  particular  group  of  strata  has  gradually 
come  to  acquire  a  chronological  meaning.  Thus  we  speak  of  the 
Oolites  or  Oolitic  formations  of  England,  and  include  under  these 
terms  a  thick  series  of  limestones,  clays,  sandstones,  and  other 
strata,  replete  with  organic  remains,  and  containing  the  records 
of  a  long  interval  of  geological  time.  But  we  also  speak  of  the 
Oolitic  period  —  a  phrase  which,  in  the  strict  grammatical  use 
of  the  word,  is  of  course  incorrect,  but  which  conveniently  desig- 
nates the  period  of  geological  time  during  which  the  great  series 
of  Oolites  was  deposited,  and  when  the  abundant  life  of  which 
they  contain  the  remains  flourished  on  the  surface  of  the  earth. 
This  chronological  meaning  has  indeed  come  to  be  the  more 
usual  sense  in  which  the  names  of  the  major  subdivisions  of  the 
Geological  Record  are  generally  employed.  Such  adjectives  as 
Devonian  and  Jurassic  do  not  so  much  suggest  to  the  mind  of 
the  geologist  Devonshire  and  the  Jura  Mountains,  from  which 
they  were  taken,  nor  even  the  rocks  to  which  they  are  applied, 
as  the  great  sections  of  the  earth's  history  of  which  these  rocks 
contain  the  memorials.  He  compares  the  Jurassic  or  Devonian 
rocks  of  one  country  with  those  of  another,  studies  the  organic 
remains  contained  in  them,  and  then  obtains  materials  for  form- 

1  For  the  meanings  of  these  names  see  Chapter  XXV. 


FOSSIL  REMAINS  233 

ing  some  conception  of  what  were  the  conditions  of  geography 
and  climate,  and  what  was  the  general  character  of  the  vegetable 
and  animal  life  of  the  globe,  during  the  periods  which  he  classes 
as  Jurassic  and  Devonian. 

SUMMARY. —  Fossils  are  the  remains  or  traces  of  plants  and 
animals  which  have  been  imbedded  in  the  rocks  of  the  earth's 
crust.  From  the  exceptional  nature  of  the  circumstances  in 
which  these  remains  have  been  entombed  and  preserved,  only  a 
comparatively  small  proportion  of  the  various  tribes  of  plants 
and  animals  living  at  any  time  upon  the  earth  is  likely  to  be 
fossilised.  Those  organisms  which  contain  hard  parts  are  best 
fitted  for  becoming  fossils.  The  original  substance  of  the  organ- 
ism may,  in  rare  cases,  be  preserved;  more  usually  the  organic 
matter  is  partially  or  wholly  removed.  Sometimes  a  mere  cast 
of  the  plant  or  animal  in  amorphous  mineral  matter  retains  the 
outward  form  without  any  trace  of  the  internal  structure.  In 
other  instances,  true  petrifaction  has  taken  place,  the  organic 
structure  being  reproduced  in  calcite,  silica,  or  other  mineral  by 
molecular  replacement. 

Fossils  are  of  the  utmost  value  in  geology,  inasmuch  as  they 
indicate  (1)  former  changes  in  geography,  such  as  the  existence 
of  ancient  land-surfaces,  lakes,  and  rivers,  the  former  extension 
of  the  sea  over  what  is  now  dry  land,  and  changes  in  the  currents 
of  the  ocean;  (2)  former  conditions  of  climate,  such  as  an  Arctic 
state  of  things  as  far  south  as  Central  France,  where  bones  of 
reindeer  and  other  Arctic  animals  have  been  found;  (3)  the 
chronological  sequence  of  geological  formations,  and,  conse- 
quently, the  succession  of  events  in  geological  history,  each  great 
group  of  strata  being  characterised  by  its  distinctive  fossils.  This 
is  the  most  important  use  of  fossils.  Having  ascertained  the 
order  of  superposition  of  fossiliferous  rocks,  that  is,  the  order  in 
which  they  were  successively  deposited,  and  having  found  what 
are  the  characteristic  fossils  of  each  subdivision,  we  obtain  a 
guide  by  which  to  identify  the  various  rock-groups  from  district 
to  district,  and  from  country  to  country.  By  means  of  the  evi- 
dence of  fossils  the  stratified  rocks  of  the  Geological  Eecord  have 
been  divided  into  sections  and  subsections,  to  which  names  are 
applied  that  have  now  come  to  designate  not  merely  the  rocks 
and  their  fossils,  but  the  period  of  geological  time  during  which 
these  rocks  were  accumulated  and  these  fossils  actually  lived. 


234  GEOLOGY 


PART  IV. 

THE  GEOLOGICAL  BECOED  OF  THE  HISTOEY  OP 
THE  EAETH. 

CHAPTEE  XVI. 

EARLIEST    CONDITIONS    OF    THE    GLOBE. 

THE  foregoing  chapters  have  dealt  chiefly  with  the  materials 
of  which  the  crust  of  the  earth  consists,  with  the  processes 
whereby  these  materials  are  produced  or  modified,  and 
with  the  methods  pursued  by  geologists  in  making  their  study  of 
these  materials  and  processes  subservient  to  the  elucidation  of 
the  History  of  the  Earth.  The  soils,  rocks,  and  minerals  be- 
neath our  feet,  like  the  inscriptions  and  sculptures  of  a  long-lost 
race  of  people,  are  in  themselves  full  of  interest,  apart  from 
the  story  which  they  chronicle ;  but  it  is  when  they  are  made  to 
reveal  the  history  of  land  and  sea,  and  of  life  upon  the  earth, 
that  they  are  put  to  their  noblest  use.  The  investigation  of  the 
various  processes  whereby  geological  changes  are  carried  on  at 
the  present  day  is  undoubtedly  full  of  fascination  for  the  student 
of  nature ;  yet  he  is  conscious  that  it  gains  enormously  in  interest 
when  he  reflects  that  in  watching  the  geological  operations  of  the 
present  day  he  is  brought  face  to  face  with  the  same  instruments 
whereby  the  very  framework  of  the  continents  has  been  piled 
up  and  sculptured  into  the  present  outlines  of  mountain,  valley, 
and  plain. 

The  highest  aim  of  the  geologist  is  to  trace  the  history  of  the 
earth.  All  his  researches,  remote  though  they  may  seem  from 
this  aim,  are  linked  together  in  the  one  great  task  of  unravelling 
the  successive  mutations  through  which  each  area  of  the  earth's 
surface  has  passed,  and  of  discovering  what  successive  races  of 
plants  and  animals  have  appeared  upon  the  globe.  The  investiga- 


EARLIEST  CONDITIONS  OF   THE   GLOBE.  235 

tion  of  facts  and  processes,  to  which  the  previous  pages  have  been 
devoted,  must  accordingly  be  regarded  as  in  one  sense  introduc- 
tory to  the  highest  branch  of  geological  inquiry.  We  have  now  to 
apply  the  methods  and  principles  already  discussed  to  the  eluci- 
dation of  the  history  of  our  planet  and  its  inhabitants.  Within 
the  limits  of  this  volume  only  a  mere  outline  of  what  has  been 
ascertained  regarding  this  history  can  be  given.  I  shall  arrange 
in  chronological  order  the  main  phases  through  which  the  globe 
seems  to  have  passed,  and  present  such  a  general  summary  of  the 
more  important  facts  regarding  each  of  them  as  may,  P  hope, 
convey  an  adequate  outline  of  what  is  at  present  known  regard- 
ing the  successive  periods  of  geological  history. 

As  the  primitive  stages  of  mankind  upon  the  earth  and  the 
early  progress  of  every  race  fade  into  the  obscurities  of  mythology 
and  archaeology,  so  the  story  of  the  primeval  condition  of  our 
globe  is  lost  in  the  dim  light  of  remote  ages,  regarding  which 
almost  all  that  is  known  or  can  be  surmised  is  furnished  by  the 
calculations  and  speculations  of  the  astronomer.  If  the  earth's 
history  could  only  be  traced  out  from  evidence  supplied  by  the 
planet  itself,  it  could  be  followed  no  further  back  than  the 
oldest  portions  of  the  earth  now  accessible  to  us.  Yet  there  can 
be  no  doubt  that  the  planet  must  have  had  a  long  history  before 
the  appearance  of  any  of  the  solid  portions  now  to  be  seen.  That 
such  was  the  case  is  made  almost  certain  by  the  traces  of  a 
gradual  evolution  or  development  which  astronomers  have  been 
led  to  recognise  among  the  heavenly  bodies.  Our  earth  being 
only  one  of  a  number  of  planets  revolving  round  the  sun,  the 
earliest  stages  of  its  separate  existence  must  be  studied  in  refer- 
ence to  the  whole  planetary  system  of  which  it  forms  a  part. 
Thus,  in  compiling  the  earliest  chapter  of  the  history  of  the 
earth,  the  geologist  turns  for  evidence  to  the  researches  of  the 
astronomer  among  stars  and  nebulas. 

In  recent  years,  more  precise  methods  of  inquiry,  and,  in  par- 
ticular, the  application  of  the  spectroscope  to  the  study  of  the 
stars,  have  gone  far  to  confirm  the  speculation  known  as  the 
Nebular  Hypothesis.  According  to  this  view,  the  orderly  related 
series  of  heavenly  bodies,  which  we  call  the  Solar  System,  existed 
at  one  time,  enormously  remote  from  the  present,  as  a  Nebula  — 
that  is,  a  cloudy  mass  of  matter,  like  one  of  those  nebulous, 
faintly  luminous  clouds  which  can  be  seen  in  the  heavens.  This 


236  GEOLOGY 

nebula  probably  extended  at  least  as  far  as  the  outermost  plane- 
tary member  of  the  system  is  now  removed  from  the  sun.  It 
may  have  consisted  entirely  of  incandescent  gases  or  vapours, 
or  of  clouds  of  stones  in  rapid  movement,  like  the  stones  that 
from  time  to  time  fall  through  our  atmosphere  as  meteorites,  and 
reach  the  surface  of  the  earth.  The  collision  of  these  stones 
moving  with  planetary  velocity  would  dissipate  them  into  vapour, 
as  is  perhaps  the  case  in  the  faint  luminous  tails  of  comets.  At 
all  events,  the  materials  of  the  nebula  began  to  condense,  and  in 
so  doing  threw  off,  or  left  behind,  successive  rings  (like  those 
around  the  planet  Saturn),  which,  in  obedience  to  the  rotation 
of  the  parent  nebula,  began  to  rotate  in  one  general  plane  around 
the  gradually  shrinking  nucleus.  As  the  process  of  condensation 
proceeded,  these  rings  broke  up,  and  their  fragments  rushed  to- 
gether with  such  force  as  not  improbably  to  generate  heat  enough 
to  dissipate  them  .again  into  vapour.  They  eventually  condensed 
into  planets,  sometimes  with  a  further  formation  of  rings,  or 
with  a  disruption  of  these  secondary  rings,  and  the  consequent 
formation  of  moons  or  satellites  round  the  planets.  The  outer 
planets  would  thus  be  the  oldest,  and,  on  the  whole,  the  coolest 
and  least  dense.  Towards  the  centre  of  the  nebula  the  heaviest 
elements  might  be  expected  to  condense,  and  there  the  high  tem- 
perature would  longest  continue.  The  sun  is  the  remaining  in- 
tensely hot  nucleus  of  the  original  nebula,  from  which  heat  is 
still  radiated  to  the  furthest  part  of  the  system. 

When  a  planetary  ring  broke  up,  and  by  the  heat  thereby 
generated  was  probably  reduced  to  the  state  of  vapour,  its  ma- 
terials, as  they  cooled,  would  tend  to  arrange  themselves  in  ac- 
cordance with  their  respective  densities,  the  heaviest  in  the  centre, 
and  the  lightest  outside.  In  process  of  time,  as  cooling  and  con- 
traction advanced,  the  outer  layers  might  grow  quite  cold,  while 
the  inner  nucleus  of  the  planet  might  still  be  intensely  hot. 
Such,  in  brief,  is  the  well-known  Nebular  Hypothesis. 

Now  the  present  condition  of  our  earth  is  very  much  what,  ac- 
cording to  this  hypothesis  or  theory,  it  might  be  expected  to  be. 
On  the  outside  comes  the  lightest  layer  or  shell  in  the  form  of  an 
Atmosphere,  consisting  of  gases  and  vapours.  Below  this  gaseous 
envelope  which  entirely  surrounds  the  globe  lies  an  inner  en- 
velope of  water,  the  ocean,  which  covers  about  two-thirds  of  the 
earth's  surface,  and  is  likewise  composed  of  gases.  Underneath 


EARLIEST  CONDITIONS  OF  THE  GLOBE  237 

this  watery  covering,  and  rising  above  it  in  dry  land,  rests  the 
solid  part  of  the  globe,  which,  so  far  as  accessible  to  us,  is  com- 
posed of  rocks  twice  or  thrice  the  weight  of  pure  water.  But 
observations  with  the  pendulum  at  various  heights  above  the  sea 
show  that  the  attraction  of  the  earth  as  a  whole  indicates  that 
the  globe  probably  has  a  density  about  five  and  a  half  times  that 
of  water.  Hence  we  may  infer  that  its  inner  nucleus  not  im- 
probably consists  of  heavy  materials,  and  may  be  metallic.  There 
is  thus  evidence  of  an  arrangement  of  the  planet's  materials  in 
successive  spherical  shells,  the  lightest  or  least  dense  being  on  the 
outside,  and  the  heaviest  or  most  dense  in  the  centre. 

Again,  the  outside  of  the  earth  is  now  quite  cool;  but  abun- 
dant proof  exists  that  at  no  great  distance  below  the  surface  the 
temperature  is  high.  Volcanoes,  hot  springs,  and  artificial  bor- 
ings all  over  the  world  testify  to  the  abundant  store  of  heat  within 
the  earth.  Probably  at  a  depth  of  not  more  than  20  miles  from 
the  surface  the  temperature  is  as  high  as  the  melting-point  of 
any  ordinary  rock  at  the  surface.  By  far  the  largest  part  of  the 
planet,  therefore,  is  hotter  than  molten  iron.  We  need  have  no 
hesitation  in  admitting  it  to  be  highly  probable  that  the  earth 
was  formerly  in  the  state  of  incandescent  vapour,  and  that  it  has 
ever  since  that  time  been  cooling  and  contracting.  Its  present 
shape  affords  strong  presumption  in  favour  of  the  opinion  that 
the  globe  was  once  in  a  plastic  condition.  The  flattening  at  the 
poles  and  bulging  at  the  equator,  or  what  is  called  the  oblately 
spheroidal  figure  of  the  planet,  is  just  the  shape  which  a  plastic 
mass  would  have  assumed  in  obedience  to  the  influence  of  the 
movement  of  rotation,  imparted  to  it  when  detached  from  the 
parent  nebula. 

At  present  a  complete  rotation  is  performed  by  the  earth  in 
twenty-four  hours.  But  calculations  have  been  made  with  the 
result  of  showing  that  originally  the  rate  of  rotation  was  much 
greater.  Fifty-seven  millions  of  years  ago  it  was  about  four 
times  faster,  the  length  of  the  day- being  only  six  and  three-quar- 
ter hours.  The  moon  at  that  time  was  only  about  35,000  miles 
distant  from  the  earth,  instead  of  239,000  miles  as  at  present. 
Since  these  early  times  the  rate  of  rotation  has  gradually  been 
diminishing,  and  the  figure  of  the  earth  has  been  slowly  tending 
to  become  more  spherical,  by  sinking  in  the  equatorial  and  rising 
in  the  polar  regions. 


238  GEOLOGY 

Of  the  first  hard  crust  that  formed  upon  the  surface  of  the  earth 
no  trace  has  yet  been  found.  Indeed,  there  is  reason  to  suppose 
that  this  original  crust  would  break  up  and  sink  into  the  molten 
mass  beneath,  and  that  not  until  after  many  such  formations  and 
submergences  did  a  crust  establish  itself  of  sufficient  strength  to 
form  a  permanent  solid  surface.  Even  though  solid,  the  surface 
may  still  have  been  at  a  glowing  red-heat,  like  so  much  molten 
iron.  Over  this  burning  nucleus  lay  the  original  atmosphere, 
consisting  -not  merely  of  the  gases  in  the  present  atmosphere, 
but  of  the  hot  vapours  which  subsequently  condensed  into  the 
ocean,  or  were  absorbed  into  the  crust.  It  was  a  hot,  vaporous 
envelope,  .under  the  pressure  of  which  the  first  layers  of  water 
that  condensed  from  it  may  have  had  the  temperature  of  molten 
lead.  As  the  steam  passed  into  water,  it  would  carry  down  with 
it  the  gaseous  chlorides  of  sodium,  magnesium,  and  other  vapours 
in  the  original  atmosphere,  so  that  the  first  ocean  was  probably 
not  only  hot,  but  intensely  saline. 

Eegarding  these  early  ages  in  the  earth's  history  we  can  only 
surmise,  for  no  direct  record  of  them  has  been  preserved.  They 
are  sometimes  spoken  of  as  pre-geological ;  but  geology  really 
embraces  the  whole  history  of  the  planet,  no  matter  from  what 
sources  the  evidence  may  be  obtained.  Deposits  from  this  orig- 
inal hot  saline  ocean  have  been  supposed  to  be  recognisable  in  the 
very  oldest  crystalline  schists ;  but  for  this  supposition  there  does 
not  appear  to  be  any  good  ground.  The  early  history  of  our 
planet,  like  that  of  man  himself,  is  lost  in  the  dimness  of  an- 
tiquity, and  we  can  only  speculate  about  it  on  more  or  less 
plausible  suppositions. 

When  we  come  to  the  solid  framework  of  the  earth  we  stand 
on  firmer  footing  in  the  investigation  of  geological  history.  The 
terrestrial  crust,  or  that  portion  of  the  globe  which  is  accessible 
to  human  observation,  has  been  found  to  consist  of  successive 
layers  of  rock,  which,  though  far  from  constant  in  their  occur- 
rence, and  though  often  broken  and  crumpled  by  subsequent 
disturbance,  have  been  recognised  over  a  large  part  of  the  globe. 
They  contain  the  earth's  own  chronicle  of  its  history,  which  has 
already  been  referred  to  as  the  Geological  Eecord,  and  the  sub- 
division of  which  into  larger  and  minor  sections,  according 
mainly  to  the  evidence  of  fossils,  was  explained  in  the  preceding 
chapter. 


EARLIEST  CONDITIONS  OF  THE  GLOBE  239 

Had  the  successive  layers  of  rock  that  constitute  the  Geological 
Kecord  remained  in  their  original  positions,  only  the  uppermost, 
and  therefore  most  recent,  of  them  would  have  been  visible,  and 
nothing  more  could  have  been  learnt  regarding  the  underlying 
layers,  except  in  so  far  as  it  might  have  been  possible  to  explore 
them  by  boring  into  them.  But  the  deepest  mines  do  not  reach 
greater  depths  than  between  3,000  and  4,000  feet  from  the  sur- 
face. Owing,  however,  to  the  way  in  which  the  crust  of  the 
earth  has  been  plicated  and  fractured,  portions  of  the  bottom 
layers  have  been  pushed  up  to  the  surface,  and  those  that  lay 
above  them  have  been  thrown  into  vertical  or  inclined  positions, 
so  that  we  can  walk  over  their  upturned  edges  and  examine  them, 
bed  by  bed.  Instead  of  being  restricted  to  merely  the  uppermost 
few  hundred  feet  of  the  crust,  we  are  enabled  to  examine  many 
thousand  feet  of  its  rocks.  The  total  mean  thickness  of  the  ac- 
cessible fossiliferous  rocks  of  Europe  has  been  estimated  at  75,000 
feet,  or  upwards  of  14  miles.  This  vast  depth  of  rock  has  been 
laid  bare  to  observation  by  successive  disturbances  of  the  crust. 

The  main  divisions  of  the  Geological  Eecord  and,  we  may  also 
say,  of  geological  time,  are  five:  (1)  Archaean,  embracing  the 
periods  of  the  earliest  rocks,  wherein  no  traces  of  organic  life 
occur;  (2)  Palaeozoic  (ancient  life)  or  Primary,  including  the 
long  succession  of  ages  during  which  the  earliest  types  of  life 
existed;  (3)  Mesozoic  (middle  life)  or  Secondary,  comprising  a 
series  of  periods  when  more  advanced  types  of  life  nourished ;  (4) 
Cainozoic  (recent  life)  or  Tertiary,  embracing  the  ages  when  the 
existing  types  of  life  appeared,  but  excluding  man;  and  (5) 
Quaternary  or  Post-tertiary  and  Eecent,  including  the  time  since 
man  appeared  upon  the  earth.  It  must  not  be  supposed  that 
each  of  these  five  divisions  was  of  the  same  duration.  The 
Palaeozoic  ages  were  probably  vastly  more  prolonged  than  those 
of  any  later  division;  while  the  Quaternary  periods  must  com- 
prise a  very  much  briefer  time  than  any  of  the  other  four  groups. 

Each  of  these  main  sections  is  further  subdivided  into  systems 
or  periods,  and  each  system  into  formations  as  already  explained. 
Arranged  in  their  order  of  sequence,  the  various  divisions  of  the 
Geological  Eecord  may  be  placed  as  in  the  accompanying  Table. 


240  GEOLOGY 


THE  GEOLOGICAL  RECORD, 

or,  Order  of  Succession  of  the  Stratified  Formations  of  the  Earth's  Crust 


Recent  and  Prehistoric. 
Pleistocene  or  Glacial. 


a 


Pliocene. 
Miocene. 
Oligocene. 


Eocene. 


Cretaceous. 
Danian. 
Senonian. 
Turonian. 
Cenomanian. 
Gault  (Albian). 
Neocomian. 

Jurassic. 
Purbeckian. 
Portlandian. 
Kimmeridgian. 
Corallian. 
Oxfordian. 
Bathonian. 
Bajocian. 
Liassic. 

Triassic. 
Rhsetic. 

Keuper  or  Upper  Trias. 
Muschelkalk. 
Bunter  or  Lower  Trias. 


EARLIEST  CONDITIONS  OF  THE  GLOBE 


241 


Permian. 

Upper  Red  Sandstones,  clays,  and  gypsum. 
Magnesian    Limestone    (Zechstein). 
Marl-Slate  (Kupferschiefer). 
Lower  Red  Sandstones,  breccias,  etc.  (Rothliegende), 

Carboniferous. 
Coal-Measures.  , 
Millstone-Grit. 
Carboniferous  Limestone  series. 

Devonian  and  Old  Red  Sandstone. 


Devonian 
Type 

Old  Red 

Sandstone 
Type 


Upper  —  Cypridina  and  Goniatite  beds. 
Middle — Stringocephalus  (Eifel)   Limestone. 
Lower  —  Spirifer  Sandstone,  etc. 

Upper  Yellow  and  Red  Sandstones,  with  Holop- 

tychius,  Pterichthys  major,  etc. 
Lower  Sandstones,  flagstones,  and  conglomerates, 

with  Cephalaspis,  Coccosteus,  Asterolepis,  etc. 


Silurian. 

Ludlow  group. 

Wenlock  group. 

Upper  Llandovery  group. 
f  Lower  Llandovery  group. 
I  Caradoc  and  Bala  group. 
1  Llandeilo  group. 
[Arenig  group. 


.a  & 

fc 
O  rt 

Upper  —  Tremadoc  Slates. 

8  | 

O       M 

a  $ 

oi  T3 

Lingula  Flags. 

11 

2  1 

Lower  —  Menevian  group. 

Oj   P-4 

P-l     H 

11 

Harlech  group. 

O 

5£ 

Longmyndian  —  Uriconian. 

[Dalradian.] 

Torridonian. 

Archaean  —  Lewisian,  Hebridean. 


THE  PRE-CAMBRIAN  PERIODS. 


Owing  to  the  revolutions  which  the  crust  of  the  earth  has  un 
dergone,  there  have  been  pushed  up  to  the  surface,  from  under 
neath  the  oldest  fossiliferous  strata,  certain  very  ancient  crystal- 


242  GEOLOGY 

line  rocks  which  form  what  is  termed  the  Archaean  system.  As 
already  mentioned,  these  rocks  have  by  some  geologists  been  sup- 
posed to  be  a  part  of  the  primeval  crust  of  the  planet,  which 
solidified  from  fusion.  By  others  they  have  been  thought  to 
have  been  formed  in  the  boiling  ocean,  which  first  condensed 
upon  the  still  hot  surface  of  the  globe.  In  truth,  we  are  still 
profoundly  ignorant  as  to  the  conditions  under  which  they  arose. 
We  have  hardly  any  means  of  ascertaining  in  what  order  they 
were  formed.  We  know  no  method  of  determining  whether  those 
of  one  region  belong  to  the  same  period  as  those  of  another.  N  or 
can  we  always  be  sure  that  what  have  been  called  Archaean  rocks 
may  not  belong  to  -a,  much  later  part  of  the  Geological  Eecord, 
their  peculiar  crystalline  structure  having  been  superinduced 
upon  them  by  some  of  those  subterranean  movements  described 
in  Chapter  XIII. 

Of  Archaean  rocks  the  most  abundant  is  gneiss,  passing  on  the 
one  hand  into  granite,  and  on  the  other  into  micaceous  and 
argillaceous  schists,  with  interstratified  bands  of  various  horn- 
blendic,  pyroxenic,  and  garnetiferous  rocks,  limestone,  dolomite, 
serpentine,  quartzite,  graphite,  hematite,  magnetite,  etc.  These 
various  materials  are  more  or  less  distinctly  bedded.  But  the 
beds  are  for  the  most  part  inconstant,  swelling  out  into 
thick  zones,  and  then  rapidly  diminishing  and  dying  out.  This 
bedding  somewhat  resembles  that  of  sedimentary  rocks,  and  the 
manner  in  which  the  limestone  and  graphite  occur,  recalls  the 
way  in  which  limestone  and  coal  are  found  in  the  fossiliferous 
formations.  The  inference  has  accordingly  been  drawn  that  the 
Archaean  crystalline  bands  were  really  deposited  as  chemical  pre- 
cipitates or  mechanical  sediments  on  the  floor  of  the  primeval 
ocean,  and  have  since  been  more  or  less  crystallised  and  disturbed. 
But  from  what  has  been  brought  forward  in  Chapter  XIII,  re- 
garding the  totally  new  structures  which  have  been  developed  in 
rocks  by  subterranean  movement,  it  is  evident  that  a  bedded  ar- 
rangement and  a  crystalline  texture,  like  those  of  the  Archaean 
gneisses  and  schists,  have  sometimes  been  induced  in  rocks  by  ex- 
cessive crumpling,  fracture,  and  shearing.  How  far,  therefore, 
the  apparent  bedding  of  Archaean  rocks  is  their  original  condi- 
tion, or  is  the  result  of  subsequent  disturbance,  is  a  question  that 
cannot  yet  be  answered. 

The  alternations  of  gneiss  and  other  crystalline  masses  form 


EARLIEST  CONDITIONS  OF  THE  GLOBE  243 

bands  which  are  usually  placed  on  end  or  at  high  angles,  and 
are  often  intensely  crumpled  and  puckered,  having  evidently  un- 
dergone enormous  crushing  (Fig.  114).  Attempts  have  been 
made  to  subdivide  them  into  groups  or  series,  according  to  their 
apparent  order  of  succession  and  lithological  characters.  But 
such  subdivisions,  even  where  practicable,  are  probably  only  of 
local  value.  As  a  rule,  those  members  of  the  system  which,  if 
the  succession  of  beds  may  be  trusted,  are  the  lowest  and  oldest, 
present  coarser  crystalline  characters  than  those  which  seem  to 
be  higher  and  later.  They  often  consist  of  massive  granitic 
gneiss,  with  abundant  veins  and  bands  of  the  coarsely  crystalline 
variety  of  granite,  known  as  pegmatite.  The  apparently  higher 
rocks  are  less  coarsely  crystalline  gneiss,  and  often  mica-schists 
and  other  schistose  masses. 


Fig.  114. —  Fragment  of  crumpled  Schist. 

No  unquestionable  relic  of  organic  existence  has  been  met  with 
among  Archaean  rocks.  Some  of  the  Archaean  limestones  of  Can- 
ada have  yielded  a  peculiar  mixture  of  serpentine  and  calcite, 
with  a  structure  which  is  regarded  by  some  able  naturalists  as 
that  of  a  reef -building  f  oraminifer.  It  occurs  in  masses,  and  is 
supposed  by  these  writers  to  have  grown  in  large,  thick  sheets  or 
reefs  over  the  sea-bottom.  By  most  observers,  however,  this 
supposed  organism  (to  which  the  name  of  Eozoon  has  been 
given)  is  now  regarded  as  merely  a  mineral  segregation,  and  vari- 
ous undoubted  mineral  structures  are  pointed  to  in  illustration 
and  confirmation  of  this  view. 

Archaean  rocks  cover  a  large  area  in  Europe.  Among  the 
Hebrides  and  along  the  north-west  coast  of  the  Scottish  High- 
lands, where  they  are  most  largely  developed,  they  consist  of  a 


244  GEOLOGY 

very  ancient  group  of  rocks,  of  which  the  most  conspicuous  are 
various  forms  of  gneiss  (Lewisian,  Hebridean,  Fundamental), 
overlain  by  a  much  younger  series  of  dull  red  sandstones,  con- 
glomerates, and  dark  gray  shales  (Torridon),  which  lie  on  the 
gneiss  unconformably,  and  are  covered  unconformably  by  the 
oldest  Cambrian  strata,  containing  annelid  tracks  and  the  trilo- 
bite  Olenellus.  The  gneiss  gives  rise  to  a  singular  type  of  scen- 
ery. Over  much  of  that  region  it  forms  hummocky  bosses  of 
naked  rock,  with  tarns  and  peat-bogs  lying  in  the  hollows,  seldom 
rising  into  mountains,  but  forming  the  platform  which  supports 
a  singular  group  of  red  (Torridon)  sandstone  mountains.  Here 
and  there,  it  mounts  up  into  solitary  hills  or  groups  of  hills.  The 
highest  point  it  reaches  on  the  mainland  is  among  the  mountains 
on  the  east  side  of  Loch  Maree,  in  Eoss-shire,  where  it  attains  an 
elevation  of  3,000  feet.  Some  of  its  masses  in  that  region  were 
mountains  at  the  time  of  the  deposition  of  the  overlying  Torridon 
sandstone,  which  when  removed  by  denudation  reveals  a  very 
ancient  system  of  hills  and  valleys.  In  the  Island  of  Harris 
the  gneiss  sweeps  upwards  into  rugged  mountainous  ground,  of 
which  the  highest  summits  rise  more  than  2,600  feet  out  of  the 
Atlantic,  and  are  visible  far  and  wide  as  a  notable  landmark. 
The  different  varieties  of  gneiss  are  associated  with  dykes  of 
various  basic  igneous  rocks,  and  bands  of  pegmatite.  Eocks  of 
similar  character  appear  likewise  in  the  west  of  Ireland ;  while  in 
Anglesey,  and  possibly  in  the  south-west  of  England,  other  scat- 
tered bosses  of  them  rise  to  the  surface. 

Later  than  the  Archaean  gneiss,  but  older  than  the  lowest  Cam- 
brian strata,  are  certain  groups  of  sedimentary  (partly  also 
igneous)  rocks  which  at  present  can  only  be  grouped  under  the 
common  term  Pre- Cambrian.  The  Torridon  sandstone  reaches 
a  thickness  of  8,000  or  10,000  feet,  and  is  almost  entirely  con- 
fined to  the  west  of  the  counties  of  Sutherland  and  Eoss.  It 
there  forms  a  remarkable  group  of  pyramidal  mountains,  to 
which  their  nearly  horizontal  stratification  gives  a  characteristic 
architectural  aspect.  Traces  of  organic  remains  (annelid  tracks, 
etc.)  have  been  found  in  these  strata. 

The  term  "  Dalradian  "  has  recently  been  applied  to  a  thick 
series  of  metamorphosed  sedimentary  and  igneous  rocks  forming 
the  Central  and  Southern  Highlands  of  Scotland.  They  must 
be  of  great  thickness,  but  their  true  geological  position  is  not  yet 


EARLIEST  CONDITIONS  OF  THE  GLOBE  245 

ascertained.  They  may  possibly  contain  altered  representatives 
of  the  old  gneiss,  Torridon  sandstone  and  Cambrian  quartzites 
and  limestones  of  the  north-west. 

On  the  borders  of  Wales  and  Shropshire  a  thick  series  of 
sedimentary  rocks  (Longmyndian)  forms  the  Longmynd  country. 
It  appears  to  be  Pre-Cambrian,  and  may  be  partly  the  equivalent 
of  the  Torridon  sandstone  of  the  north-west.  It  is  underlain  by 
a  group  of  felsitic  lavas  and  tuffs  named  Uriconian. 

On  the  continent  of  Europe,  Archaean  rocks  have  their  greatest 
extension  in  Scandinavia,  where  they  evidently  belong  to  the 
same  ancient  land  as  that  of  which  the  Hebrides  and  Scottish 
Highlands  are  fragments.  They  range  through  Finland  far  into 
Russia,  appearing  in  the  centre  of  the  chain  of  the  Ural  Moun- 
tains. They  form  likewise  the  nucleus  of  the  Carpathians  and 
the  Alps,  and  appear  in  detached  areas  in  Bavaria,  Bohemia, 
France,  and  the  Pyrenees.  They  are  estimated  to  occupy  an  area 
of  more  than  2,000,000  of  square  miles  in  the  more  northerly 
part  of  North  America,  stretching  from  the  Arctic  regions  south- 
wards to  the  great  lakes.  Both  in  the  Old  and  New  World,  the 
Archaean  rocks  are  chiefly  exposed  in  the  northern  tracts  of  the 
continents.  The  areas  which  they  there  overspread  were  proba- 
bly land  at  an  early  geological  period,  and  it  was  the  waste  of 
this  land  that  mainly  supplied  the  original  materials  out  of  which 
the  enormous  masses  of  stratified  rocks  were  formed.  Various 
thick  accumulations  of  sedimentary  and  igneous  rocks  have  been 
ascertained  to  lie  in  North  America,  as  in  Europe,  between  the 
Archaean  gneisses  and  the  base  of  the  Cambrian  system. 

In  the  southern  hemisphere  also  ancient  gneisses  and  other 
schists  rise  from  under  the  oldest  fossiliferous  formations.  In 
Australia  and  in  New  Zealand  they  cover  large  tracts  of  country, 
and  appear  in  the  heart  of  the  mountain  ranges.  It  thus  appears 
that  all  over  the  world  the  oldest  known  rocks  are  gneisses 
and  similar  or  allied  crystalline  masses,  having  a  remarkable 
uniformity  of  character. 


246  GEOLOGY 


CHAPTEE  XVII. 

PALAEOZOIC    PERIODS  —  CAMBRIAN. 

THE  portion  of  geological  history  which  treats  of  those  ages 
in  which  the  earliest  known  types  of  plants  and  animals 
lived  is  termed  Palaeozoic.  Of  the  first  appearance  of 
organic  life  upon  our  planet  we  know  nothing.  Whether  plants 
or  animals  came  first,  and  in  what  forms  they  came,  are  ques- 
tions to  which  as  yet  no  satisfactory  answer  can  be  given.  The 
oldest  discovered  fossils  are  assuredly  not  vestiges  of  the  first 
living  things  that  peopled  the  globe.  There  is  every  reason,  in- 
deed, to  hope  that  as  researches  in  all  parts  of  the  world  are 
pushed  into  older  and  yet  older  rocks,  still  more  ancient  organ- 
isms may  be  discovered.  But  it  is  in  the  highest  degree  im- 
probable that  any  trace  of  the  earliest  beginnings  of  life  will  ever 
be  found.  The  first  plants  and  the  first  animals  were  probably 
of  a  lowly  kind,  with  no  hard  parts  capable  of  preservation  in 
the  fossil  state.  Moreover,  the  sedimentary  rocks  which  may 
have  chronicled  the  first  advent  of  organised  existence  are  hardly 
likely  to  have  escaped  the  varied  revolutions  to  which  all  parts  of 
the  crust  of  the  earth  have  been  exposed.  They  have  more  prob- 
ably been  buried  out  of  sight,  or  have  been  so  crushed,  broken, 
and  metamorphosed,  that  their  original  condition,  together  with 
any  fossils  they  may  have  enclosed,  is  no  longer  to  be  recognised. 
The  first  chapters  have  been,  as  it  were,  torn  out  from  the 
chronicle  of  the  earth's  history. 

The  Palaeozoic  rocks,  which  contain  the  earliest  record  of 
plant  and  animal  life,  consist  mainly  of  the  hardened  mud,  sand, 
and  gravel  of  the  sea-bottom.  Here  and  there  they  include  beds, 
or  thick  groups  of  beds,  of  limestone  composed  of  marine  shells, 
crinoids,  corals,  and  other  denizens  of  salt  water.  They  are  thus 
essentially  the  chronicles  of  the  sea.  But  they  also  contain 
occasional  vestiges  of  shores,  and  even  of  the  jungles  and  swamps 


PALAEOZOIC  PERIODS  —  CAMBRIAN  247 

of  the  land,  with  a  few  rare  glimpses  into  the  terrestrial  life  of  the 
time.  Everywhere  they  abound  in  evidence  of  shallow  water; 
for  though  chiefly  marine,  they  appear  to  have  been  accumulated 
not  far  from  land.  We  may  believe  that  in  the  earliest  periods, 
as  at  the  present  day,  the  sediment  washed  away  from  the  land 
has  been  deposited  on  the  sea-floor,  for  the  most  part  at  no  great 
distance  from  the  coast. 

The  land  from  the  waste  of  which  the  Palaeozoic  rocks  were 
formed  lay  in  Europe  and  North  America  chiefly  towards  the 
north.  It  no  doubt  consisted  of  Archa3an  rocks,  such  as  still 
rise  out  from  under  the  oldest  Paleozoic  formations.  As  already 
mentioned,  the  north-west  Highlands  of  Scotland,  part  of  the 
table-land  of  Scandinavia,  and  most  of  North  America  to  the 
north  of  the  great  lakes,  are  probably  portions  of  that  earliest 
land,  which,  after  being  deeply  buried  under  later  geological  ac- 
cumulations, have  once  more  been  laid  bare  to  the  winds  and 
waves.  We  can  form  some  conception  of  the  bulk  of  the  primeval 
northern  land  by  noting  the  thickness  of  sedimentary  rocks  that 
were  formed  out  of  its  detritus  during  the  Palaeozoic  periods. 
The  older  half  of  the  Palaeozoic  rocks  in  the  British  Islands,  for 
example,  is  at  least  16,000  feet  or  3  miles  thick,  and  covers  an 
area  of  not  less  than  60,000  square  miles.  This  material,  derived 
from  the  waste  of  the  Archaean  rocks,  would  make  a  table-land 
larger  than  Spain,  with  an  average  height  of  5,000  feet,  or  a 
mountain  chain  1,800  miles  long,  with  an  average  elevation  of 
16,000  feet.  Of  the  general  form  and  height  of  the  northern 
land  that  supplied  this  vast  mass  of  sedimentary  matter  nothing 
is  known.  Perhaps  it  was  lofty;  but  it  may  have  been  slowly 
uplifted,  so  that  its  rise  compensated  for  the  ceaseless  degrada- 
tion of  its  surface. 

Abundant  evidence  of  volcanic  action  has  been  preserved 
among  the  Palaeozoic  rocks  in  the  form  of  piles  of  lavas  and  tuffs. 
We  find  also  many  indications  of  upward  and  downward  move- 
ments of  the  crust  of  the  earth.  The  mere  fact  of  the  super- 
position of  many  thousands  of  feet  of  shallow-water  strata,  one 
above  another,  is  a  proof  of  gradual  subsidence.  For  it  is  evi- 
dent that  the  accumulation  of  such  a  thickness  of  sediment,  and 
the  continuance  of  a  shallow  sea  over  the  area  of  deposition,  could 
only  take  place  during  a  progressive  subsidence. 

The  life  of  the  Palaeozoic  periods,  so  far  as  known  from  the 


248  GEOLOGY 

fossils  which  have  been  obtained  from  the  rocks,  appears  to  have 
been  far  more  uniform  over  the  whole  globe  than  at  any  subse- 
quent epoch  in  geological  history.  For  instance,  the  same  species 
of  fossils  are  found  in  corresponding  rocks  in  Britain,  Eussia, 
United  States,  China,  and  Australia,  The  climate  of  the  globe 
at  that  ancient  date  was  doubtless  more  uniform  than  it  after- 
wards became,  and  was  probably  also  generally  warmer.  Palae- 
ozoic fossils,  obtained  from  high  northern  latitudes,  are  precisely 
similar  to  those  that  abound  in  England,  whence  it  may  be  in- 
ferred that  not  only  was  there  a  greater  uniformity  of  climate, 
but  that  the  great  cold  which  now  characterises  the  Arctic  regions 
did  not  then  exist. 

In  the  earlier  Palaeozoic  periods,  the  animal  life  of  the  globe 
appears  to  have  been  entirely  invertebrate,  the  highest  known 
types  being  chambered  shells,  of  which  our  living  nautilus  is  a 
representative.  In  the  middle  periods  vertebrate  life  appeared. 
The  earliest  known  vertebrate  forms  are  fishes  akin  to  some 
modern  sharks  and  to  the  sturgeon,  the  polypterus  of  the 
Nile,  and  the  gar-pike  of  American  lakes.  The  most  highly 
organised  forms  of  existence  upon  the  earth's  surface  in  the 
later  Palaeozoic  periods  were  amphibians  —  a  class  of  animals 
represented  at  the  present  day  by  frogs,  toads,  newts,  and 
salamanders.  It  is  evident,  however,  that  the  number  and 
kinds  of  animal  remains  preserved  in  Palaeozoic  rocks  afford 
only  an  imperfect  record  of  the  animal  life  of  these  early  ages. 
Whole  tribes  of  creatures  no  doubt  existed  of  which  no  trace 
whatever  has  yet  been  recovered.  An  accidental  discovery  may 
at  any  moment  reveal  the  former  presence  of  some  of  these  van- 
ished forms.  For  example,  the  examination  of  a  fossil  tree- 
trunk  imbedded  among  the  coal-strata  of  Nova  Scotia,  led  to  the 
finding  of  the  first  and  as  yet  almost  the  only  traces  of  Palaeozoic 
land-shells,  though  thousands  of  species  of  marine  shells,  belong- 
ing to  the  same  period,  had  long  been  known.  Every  year  is  en- 
larging our  knowledge  in  these  respects,  but  from  the  very  nature 
of  the  circumstances  in  which  the  records  of  the  rocks  were 
formed,  we  cannot  expect  this  knowledge  ever  to  be  more  than 
fragmentary. 

The  Palaeozoic  rocks  are  divided  into  five  systems  which  in 
the  order  of  their  age  have  been  named;  (1)  Cambrian;  (2) 
Silurian;  (3)  Devonian;  (4)  Carboniferous;  (5)  Permian. 


PALAEOZOIC  PERIODS  —  CAMBRIAN  249 

CAMBRIAN. 

The  strata  containing  the  earliest  organic  remains  were  for- 
merly known  as  Graywacke,  from  the  rock  which  is  specially 
abundant  among  them.  They  were  also  termed  Transition,  from 
the  supposition  that  they  were  deposited  during  a  transitional 
period,  between  the  time  when  no  organic  life  was  possible  on  the 
earth's  surface  and  the  time  when  plant  and  animal  life  abounded. 
But  the  late  Sir  Koderick  Murchison,  who  first  explored  them, 
showed  that  they  contain  a  series  of  formations,  each  character- 
ised by  its  own  assemblage  of  organic  remains.  He  called  them 
the  Silurian  system,  after  the  name  of  the  old  British  tribe  —  the 
Silures,  who  lived  on  the  borders  of  England  and  Wales,  where 
these  rocks  are  especially  well  developed.  This  name  has  now 
been  adopted  all  over  the  world  as  the  designation  of  those 
stratified  formations  which  contain  the  same  or  similar  organic 
remains  to  those  found  in  the  typical  region  described  by 
Murchison. 

While  the  succession  of  the  rocks  and  fossils  was  established  by 
that  geologist  in  South  Wales,  and  in  the  border  counties  of 
Wales  and  England,  Professor  Sedgwick  was  at  work  among  sim- 
ilar rocks  in  North  Wales.  These  were  at  first  believed  to  be 
all  older  than  those  called  Silurian,  and  were  accordingly  named 
"CAMBRIAN,  after  the  old  name  for  Wales,  Cambria.  In  the  end, 
however,  it  was  found  that  throughout  a  large  part  of  them  the 
same  fossils  occurred,  as  in  the  Silurian  series,  and  they  were 
accordingly  claimed  as  Silurian.  Much  controversy  has  since 
been  carried  on  regarding  the  limits  and  names  to  be  assigned  to 
these  rocks,  and  geologists  are  not  yet  agreed  upon  the  nomen- 
clature that  should  be  followed.  Murchison  and  his  followers 
claimed  the  Cambrian  as  the  lowest  portion  of  the  Silurian  sys- 
tem, while  Sedgwick  and  his  disciples  maintained  that  the  lower 
half  of  the  Silurian  system  should  be  included  in  his  Cambrian 
series.  There  can  be  no  doubt  that  the  first  succession  of  organic 
remains  established  among  these  ancient  members  of  the  great 
Paleozoic  series  of  formations  was  that  worked  out  by  Murchison 
and  named  by  him  Silurian.  But  it  has  been  found  convenient  to 
retain  the  name  Cambrian  for  the  oldest  group  of  fossiliferous 
formations.  It  may  be  well  to  repeat  that  these  words,  like  all 
those  adopted  by  geologists  to  distinguish  the  successive  rock- 


250  GEOLOGY 

groups  of  the  earth's  crust,  have  acquired  a  chronological  mean- 
ing. We  speak,  not  only  of  Cambrian  and  Silurian  strata  and 
Cambrian  and  Silurian  fossils,  but  of  Cambrian  and  Silurian 
time.  The  terms  are  used  to  denote  those  particular  periods  in 
the  history  of  the  earth  when  Cambrian  and  Silurian  strata  were 
respectively  deposited,  and  when  Cambrian  and  Silurian  fossils 
were  the  living  denizens  of  sea  and  land. 

The  rocks  of  which  the  Cambrian  system  is  composed,  like 
those  of  the  whole  of  the  Lower  Palaeozoic  formations,  present 
considerable  uniformity  over  the  whole  globe.  They  consist  of 
gray  and  reddish  grits,  sandstones,  graywackes,  quartzites,  and 
conglomerates,  with  thick  groups  of  shale,  slate,  or  phyllite. 
These  sedimentary  accumulations  attain  a  great  thickness  in  some 
countries.  In  Wales  they  have  been  estimated  by  some  observers 
to  be  at  least  20,000  feet  in  depth.  Their  ripple-marks,  pebble- 
beds,  and  frequent  alternations  of  coarse  and  fine  sediment,  point 
to  their  having  probably  been  laid  down  in  comparatively  shallow 
water,  during  a  period  of  prolonged  subsidence  of  the  sea-bottom. 
Tfyey  include  tuffs  and  basic  lavas  which  indicate  contemporane- 
ous submarine  eruptions. 

With  regard  to  the  occurrence  of  fossils  among  the  older 
Pakeozoic  formations,  and  indeed  among  stratified  rocks  in  gen- 
eral, it  is  worthy  of  notice  that  they  are  far  from  being  equally 
distributed ;  that,  'on  the  contrary,  they  occur  by  preference  in 
certain  kinds  of  material  rather  than  in  others.  Grits  and 
sandstones,  for  instance,  are  comparatively  unfossiliferous,  while 
fine  shales,  slates,  and  limestones  are  often  crowded  with  fossils. 
It  is  not  that  life  was  probably  on  the  whole  more  abundant  at 
the  time  of  the  deposition  of  some  kinds  of  strata,  but  that  the 
local  conditions  for  its  growth  and  for  the  subsequent  entomb- 
ment and  preservation  of  its  remains  were  then  more  favourable. 
At  the  present  time,  for  example,  dredging  operations  show  the 
most  remarkable  variations  between  different  and  even  adjacent 
parts  of  the  sea-bottom  as  regards  the  abundance  of  marine  life. 
Some  tracts  are  almost  lifeless,  while  others  are  crowded  with  a 
varied  and  prolific  fauna.  We  can  easily  understand  that  if,  from 
the  nature  of  the  bottom,  plants  and  smaller  animals  cannot 
flourish  on  a  particular  tract,  the  larger  kinds  that  feed  on  them 
will  also  desert  it.  Even  if  organisms  live  and  die  in  some  num- 
bers over  a  part  of  the  sea-bed,  the  conditions  may  not  be  suita- 


PALAEOZOIC  PERIODS  —  CAMBRIAN  251 

ble  there  for  the  preservation  of  their  remains.  The  rate  of 
deposit  of  sediment,  for  instance,  may  be  so  slow  that  the  remains 
may  decay  before  there  is  time  for  them  to  be  covered  up ;  or  the 
sediment  may  be  unfitted  for  effectually  preserving  them,  even 
when  they  are  buried  in  it.  We  must  not  lose  sight  of  these  facts 
in  our  explorations  of  the  Geological  Eecord.  A  relation  has 
always  existed  between  the  abundance  or  absence  of  fossils  in  a 
sedimentary  rock  and  the  circumstances  under  which  the  rock 
was  originally  formed. 

The  oldest  fossiliferous  strata  (Cambrian  or  Primordial)  con- 
tain a  remarkable  assemblage  of  animal  remains,  which,  being  the 
earliest  traces  of  the  animal  life  of  the 
globe,  might  have  been  anticipated  to  belong 
to  the  very  lowest  tribes  of  the  animal  king- 
dom. But  they  are  by  no  means  of  such 
humble  organisation.  On  the  contrary,  they 
include  no  representatives  of  many  of  the 
groups  of  simpler  invertebrates,  which  we 
may  be  sure  were  nevertheless  living  at  the 
same  time.  Not  only  so,  but  some  of  the 
fossils  belong  to  comparatively  high  grades 
injthe  scale  of  invertebrate  life,  such  as 
chambered  molluscs.  From  this  incom- 
pleteness, and  from  the  wide  differences  in 
the  organic  grade  of  the  forms  actually  pre- 
served in  the  rocks,  we  may  reasonably  infer 
that  only  a  most 'meagre  representation  of 
the  life  of  that  time  has  come  down  to  us  in  Fig.  115. —  Fucoid-like 
the  fossil 'state.  Some  of  the  fossils,  more-  'Tfn^Tinniwum) 
over,  have  been  so  indistinctly  preserved  from  Cambrian 
that  considerable  difficulty  is  experienced  in 
deciding  to  what  sections  of  the  animal  or  vegetable  kingdoms 
they  should  be  assigned. 

Among  the  markings  which  have  given  rise  to  much  discus- 
sion allusion  may  be  made  to  plant-like  impressions  some  of 
which,  like  Eophyton  (Fig.  115),  have  been  claimed  as  sea-weeds. 
Others,  however,  may  only  be  irregular  wrinklings  of  the  sur- 

1  The  fractional  numbers  inserted  within  parentheses  in  the  titles  of 
the  figures  of  fossils  indicate  how  much  the  figures  have  been  reduced 
or  magnified.  Thus  J  —  reduced  one-third ;  f  —  magnified  four  times. 


252  GEOLOGY 

faces  of  deposit,  and  of  no  organic  origin  at  all  (see  p.  259). 
Another  puzzling  impression  is  that  called  Oldhamia  (Fig.  116), 
which  has  been  variously  referred  to  the  Hydrozoa,  the  Sertularia, 
the  Polyzoa,  and  the  calcareous  Alga3. 


Fig.   116. —  Oldhamia  radiata    (natural  size),  Ireland. 

Some  of  the  most  characteristic  older  Palaeozoic  organisms 
belong  to  the  Hydrozoa,  and  are  embraced  under  the  general  title 
of  Graptolites  —  a  name  given  to  them  from  their  fancied  re- 
semblance to  quill-pens.  They  were  conrp&sed  of  a  horny  or 
chitinous  substance,  and  hence  they  commonly  present  themselves 


Fig.  117. —  Hydrozoon  from  the  Cambrian  rocks   (Dictyograptus   (Dicty- 
onema)   sociale),  natural  size. 

merely  as  black  streaks  upon  the  stone.  Each  graptolite  was  a 
colony  comprising  many  individuals  which  occupied  each  its  own 
cell.  The  cells  are  in  some  kinds  placed  in  a  row  on  one  side  of 
a  supporting  rod  or  axis ;  in  other  kinds  there  is  a  row  of  cells  on 


PALAEOZOIC  PERIODS  —  CAMBRIAN 


253 


both  sides  (see  Fig.  121).  Some  varieties  are  straight,  others 
curved  or  spiral.  Some  are  simple  branches,  others  are  com- 
posed of  two  or  more  branches,  while  in  certain  types  a  large 
number  of  separate  branches  is  united  in  one  common  centre. 
One  of  the  most  ancient  hydrozoa  is  Dictyograptus  (Dictyonema, 
Fig.  117) —  a  characteristic  fossil  of  the  Cambrian  rocks  of  Scan- 
dinavia. The  graptolites  are  more  especially  characteristic  of 
the  Silurian  system. 

The  Echinodermata  had  their  representatives  in   Cambrian 
time,  though  the  remains  of  these  are  few  and  infrequent.     The 


Fig.  118. —  Cambrian  Trilobites.  (a) Paradoxides  bohemicus  (natural 
size)  ;  (6)  Agnostus  princeps  (  f  )  ;  (c)  Olenus  micrurus  (natural 
size)  ;  (d)  Ellipsocephalus  Eofti  (natural  size). 

great  tribe  of  the  Crinoids  or  Sea-lilies  (p.  261)  had  already 
established  itself  on  the  floor  of  the  Cambrian  sea,  where  also 
there  were  representatives  of  Cystideans  and  star-fishes. 

Numerous  kinds  of  Sea-worms  (Annelids)  crawled  over  the 
sandy  and  muddy  bottom  and  shores  of  the  Cambrian  ocean. 
These  creatures  have  left  no  trace  of  their  bodies,  which,  like 
those  of  their  representatives  in  the  present  ocean,  were  soft  and 
unfitted  for  preservation.  But  the  burrows  they  made  in  wet  sand 


254  GEOLOGY 

or  mud,  and  the  trails  they  left  upon  the  soft  surfaces  over  which 
they  moved,  have  been  abundantly  preserved  (see  Fig.  124). 
These  markings  afford  unquestionable  proof  of  the  presence  of 
creatures  which  have  otherwise  utterly  disappeared. 

Among  the  most  abundant  and  characteristic  fossils  of  the 
older  stratified  rocks  of  the  earth's  crust  are  those  to  which  the 
general  name  of  Trilobites  has  been  given.  These  long-extinct 
animals  were  crustaceans,  having  a  more  or  less  distinctly 
three-lobed  body,  at  one  end  of  which  was  the  head  or  cephalic 
shield,  usually  with  a  pair  of  fixed  compound  eyes ;  at  the  other 
end  the  caudal  shield  or  tail ;  while  between  the  two  shields  was 
the  ringed  or  jointed  body,  the  rings  of  which  were  movable,  so 
that  the  animal  could  bring  the  two  shields  together  or  coil  itself 
up.  It  will  be  seen  from  the  different  genera  represented  in  Figs. 
118  and  125  how  varied  were  the  forms  which  they  assumed.  In 
the  shapes  and  relative  sizes  of  the  shield  and  segmented  body, 
in  the  number  of  the  body-rings,  in  the  development  of  spines, 
and  in  other  features,  the  most  wonderful  variety  is  traceable 
among  the  trilobites  even  of  the  oldest  f ossiliferous  strata.  Some 
of  the  earliest  genera  were  also  the  largest;  Paradoxides  some- 
times reaching  a  length  of  nearly  two  feet.  Yet  contemporane- 
ous with  this  large  creature  were  some  diminutive  forms.  A 
few  genera  (among  them  Agnostm)  were  blind;  but  most  pos- 
sessed eyes  furnished  with  facets,  which  in  some  forms  are  four- 
teen in  number,  while  in  others  they  are  said  to  amount  to  15,000. 
The  peculiar  crescent-shaped  eye  on  each  side  of  the  head  is  well 
shown  in  some  of  the  forms  represented  in  Figs.  118  and  125. 
The  trilobites  appear  to  have  particularly  swarmed  on  sandy  and 
muddy  bottoms,  for  their  remains  are  abundant  in  many  sand- 
stones and  shales. 

Another  form  of  crustacean  life  represented  in  the  early  Palae- 
ozoic ocean  was  that  of  the  Phyllopods  —  animals  furnished  with 
bivalve  shell-like  carapaces,  which  protected  the  head  and  upper 
part  of  the  body,  while  the  jointed  tail  projected  beyond  it. 
Most  of  them  were  of  small  size  (see  Fig.  126).  The  character- 
istic Cambrian  genus  is  Hymenocaris. 

Of  all  the  divisions  of  the  animal  kingdom  none  is  so  inv 
portant  to  the  geologist  as  that  of  the  Mollusca.  When  one 
walks  along  the  shores  of  the  sea  at  the  present  time,  by  far  the 
most  abundant  remains  of  the  marine  organisms  to  be  there  ob- 


PALAEOZOIC  PERIODS  —  CAMBRIAN  255 

served  are  shells.  They  occur  in  all  stages  of  freshness  and  de- 
cay, and  we  may  trace  even  their  comminuted  fragments  forming 
much  of  the  white  sand  of  the  beach.  So  in  the  geological  forma- 
tions, which  represent  the  shores  and  shallow  sea-bottoms  of 
former  periods,  it  is  mainly  remains  of  the  marine  shells  that 
have  been  preserved.  From  their  abundance  and  wide  diffusion, 
they  supply  us  with  a  basis  for  the  comparison  of  the  strata  of 
different  ages  and  countries,  such  as  no  other  kind  of  organic  re- 
mains can  afford. 

It  is  interesting  and  important  to  find  that  among  the  fossils 
of  the  oldest  fossiliferous  rocks  the  remains  of  molluscan  shells 
occur,  and  that  they  are  of  kinds  which  can  be  satisfactorily  re- 
ferred to  their  place  in  the  great  series  of  the  Mollusca.  The  most 
abundant  of  them  are  representatives  of  the  Brachiopods  or 
Lamp-shells.  Among  these  are  species  of  the  genera  Lingula  (Lin~ 
gulella,  Fig.  119)  and  Discina  which  have  a  peculiar  interest,  in- 
asmuch as  they  are  the  oldest  known  molluscs, 
and  are  still  represented  by  living  species  in 
the  ocean.  They  have  persisted  with  but  little 
change  during  the  whole  of  geological  time, 
from  the  early  Palaeozoic  periods  downwards, 
for  the  living  shells  do  not  appear  to  indicate  Fig.  119^Cambri- 
any  marked  divergence  from  the  earliest  forms.  ^(Lingul^la^^a- 
They  possess  horny  shells  which  are  not  hinged  visii,  natural 
together  by  teeth.  A  more  highly  organised 
order  of  brachiopods  possesses  two  hard  calcareous  shells 
articulated  by  teeth  on  the  hinge-line.  '  These  forms, 
apparently  later  in  their  advent,  soon  vastly  outnumbered 
the  horny  lingulids  and  discinids.  So  abundant  are  they  both  in 
individuals  and  in  genera  and  species  among  the  older  Palaeozoic 
rocks,  that  the  period  to  which  these  rocks  belong  is  sometimes 
spoken  of  as  the  "  Age  of  Brachiopods." 

The  ordinary  bivalve  shells  or  Lamellibranchs  had  their  re- 
presentatives even  in  Cambrian  times.  From  that  early  period 
they  have  gradually  increased  in  numbers,  till  they  have  attained 
their  maximum  at  the  present  time.  Among  the  known  Cam- 
brian genera  are  Ctenodonta  allied  to  the  living  "  ark-shells/'  and 
Modiolopsis,  probably  representing  some  of  the  modern  mussels. 

The  Gasteropods  or  common  univalve  shells,  now  so  abundant 
in  the  ocean,  made  their  advent  not  later  than  Cambrian  time, 


256  GEOLOGY 

for  the  remains  of  the  genus  Bellerophon  (Fig.  129)  are  found 
in  the  group  of  strata  known  as  the  Lingula-flags  in  Wales. 

The  highest  division  of  the  molluscs,  the  Cephalopods,  to 
which  the  living  nautilus  and  cuttle-fish  belong,  is  but  poorly  re- 
presented at  the  present  time.  But  during  the  Palaeozoic  and 
Secondary  periods  it  flourished  exuberantly,  both  as  regards 
number  of  individuals  and  variety  of  forms.  It  is  divisible  into 
two  great  families.  In  one  of  these  the  shell  is  usually  internal 
and  is  never  chambered;  in  the  other  the  shellis  chambered  and 
external,  the  chambers  being  connected  by  a  tube  or  siphuncle. 
The  former  family  includes  all  the  living  cuttle-fishes,  squids,  and 
the  paper-nautilus;  th-s  latter  comprises  only  one  living  repre- 
sentative —  the  pearly  nautilus.  It  is  to  the  family  of  chambered 
cephalopods  that  the  Palaeozoic  forms  are  all  referable.  In  some 
the  shell  was  straight,  in  others  it  was  variously  curved.  Only 
scanty  traces  of  cephalopodan  life  have  yet  been  found  among 
the  Cambrian  rocks.  But  occasional  examples  of  the  important 
genus  Orthoceras  (see  Fig.  130)  show  that  this  great  division  of 
the  molluscs  had  even  in  the  earliest  Palaeozoic  ages  appeared 
upon  the  earth. 

As  the  term  Cambrian  denotes,  the  rocks  to  which  this  name 
is  applied  are  well  developed  in  Wales.  There,  and  in  the  bor- 
der English  counties,  they  attain  a  depth  of  perhaps  more  than 
20,000  feet.  They  are  found  also  in  the  east  of  Ireland,  while 
in  the  north-west  of  Scotland  they  appear  to  be  represented  by 
massive  red  sandstones  and  conglomerates.  The  peculiar  Pri- 
mordial fauna  has  been  widely  recognised  in  both  hemispheres. 
It  occurs  at  intervals  from  France  to  Russia,  and  from  Sweden 
to  Bohemia  and  Sardinia.  It  is  well  represented  in  the  United 
States  and  Canada,  and  it  is  met  with  even  as  far  east  as  China. 

The  following  Table  gives  the  commonly  accepted  subdivisions 
of  the  Cambrian  rocks  in  Britain. 


Tremadoc  group  —  dark  gray  slates. 
Lingula     Flags  — bluish     and     black 
flags,  and  sandstones. 


W     OJ 

W>  ~ 


PALAEOZOIC  PERIODS  —  CAMBRIAN 


257 


* 


Menevian  group  —  sandstones,  shales,  slates, 
and  grits. 

Harlech  group  —  purple,  red,  and  gray 
flags,  sandstones,  slates,  and  conglomerates. 
(Horizon  of  Olenellus.) 


Recently  the  existence  of  Olenellus  —  a  genus  of  trilobite  characteris- 
tic of  the  lowest  platform  of  the  Cambrian  system  has  been  found  in  the 
midland  counties  of  England  and  in  the  north-west  Highlands  of  Scot- 
land. In  the  latter  region  the  occurrence  of  this  fossil  fixes  the  strati- 
graphical  position  of  the  well-known  quartzites  as  Lower  Cambrian. 


258  GEOLOGY 


CHAPTER  XVIII. 

THE  SILURIAN   PERIOD. 

THE  origin  and  use  of  the  term  SILURIAN"  have  already  been 
fully  explained.  The  rocks  embraced  under  this  term 
form  a  mass  of  strata  which  in  some  countries  (Wales  and 
Scotland)  must  be  many  thousand  feet  thick.  Like  the  Cam- 
brian system  below,  into  which  they  graduate  downward,  they 
consist  mainly  of  graywackes,  sandstones,  shales,  or  slates;  but 
they  are  marked  by  the  occasional  occurrence  of  bands  of  lime- 
stone —  a  rock  which  from  this  part  of  the  geological  record  ap- 
pears in  increasing  quantity  onwards  to  recent  times.  Some 
highly  characteristic  bands  of  dark  carbonaceous  shale  are  in 
some  countries  persistent  for  long  distances,  and  contain  abun- 
dant graptolites.  Not  infrequently  these  dark  shales  are  full  of 
pyritous  impregnations,  which,  when  the  rock  weathers,  give  rise 
to  the  efflorescence  of  alum  or  the  formation  of  chalybeate 
springs ;  such  bands  are  sometimes  called  alumschists.  In  Wales, 
the  Lake  District  of  the  north  of  England,  and  to  a  less  degree  in 
the  south  of  Scotland,  there  are  remains  of  submarine  volcanic 
eruptions  of  Silurian  time  in  the  form  of  intercalated  sheets  of 
tuff  and  beds  of  different  lavas. 

In  certain  regions  (Russia,  New  York)  Silurian  rocks  have 
undergone  little  change  since  the  time  of  their  deposition ;  but, 
as  a  rule,  they  have  been  more  or  less  indurated,  plicated,  and 
dislocated  (Wales,  Lake  District,  etc.),  while  in  some  countries 
(Norway,  Scotland)  they  have  been  so  crushed  and  metamor- 
phosed as  to  have  assumed  the  character  of  schistose  rocks 
(phyllites,  mica-schists,  etc.). 

Murchison  subdivided  his  Silurian  system  into  two  great  sec- 
tions, Lower  and  Upper.  This  classification  still  holds,  though 
the  limits  and  nomenclature  of  the  several  component  groups 
have  not  been  exactly  maintained.  The  arrangement  of  the 


THE  SILURIAN  PERIOD  259 

various  subdivisions,  as  followed  in  Britain,  is  shown  in  the  table 
in  Chapter  XVIII. 

Taking  the  fossils  of  the  Silurian  system  as  a  whole,  we  find 
that  they  prolong  and  amplify  the  peculiar  type  of  life  found 
to  characterise  the  Cambrian  system.  They  include  both  flora 
and  fauna.  The  flora,  however,  is  exceedingly  meagre.  It  con- 
sists almost  entirely  of  sea-weeds,  which  occur  usually  in  the 
form  of  fucoid-like  impressions.  But,  as  already  remarked  in 
reference  to  the  so-called  plants  of  the  Cambrian  rocks,  many 
of  the  supposed  vegetable  remains  are  almost  certainly  not  such. 
Some  of  the  supposed  remains  may  be  tracks  left  by  worms, 
crustaceans,  or  other  marine  creeping  or  crawling  creatures, 
upon  soft  mud  or  sand;  others 
may  be  casts  of  hollows  made 
by  trickling  water  or  yielding 
sediment;  while  others  seem  to 
be  the  result  of  some  peculiar 
crumpling  or  puckering  of  the 
strata.  But  undoubted  remains 
of  sea-weeds  do  occur.  Some 
of  these  are  delicate  branching 
forms,  like  some  still  living,  as 
shown  in  the  organism  figured 
in  Fig.  120  from  the  Upper  Fig.  120.— An  Upper  Silu- 
Silurian  series.  Among  the  ria?.  ?7e.a-weed  (Chondrites 
TT  ci-i  •  vensvnilis) ,  natural  size. 

Upper  Silurian  strata,  also,  traces 

of  land-vegetation  have  been  detected  in  the  form  of  spores  and 
stems  of  cryptogamous  plants.  Lycopods  or  club-mosses  and 
ferns  appear  to  have  been  the  chief  types  in  the  earliest  ter- 
restrial floras;  at  least,  it  is  remains  referable  to  them  that 
chiefly  occur  in  the  older  Palaeozoic  rocks.  They  reached  a  great 
development  in  the  Carboniferous  period,  in  the  account  of 
which  a  fuller  description  of  them  will  be  given.  We  can  dimly 
picture  the  Silurian  land  with  its  waving  thickets  of  fern,  above 
which  lycopod  trees  raised  their  fluted  and  scarred  stems,  threw 
out  their  scaly  moss-like  branches,  and  shed  their  spiky  cones. 

The  fauna  of  the  Silurian  period  has  been  more  abundantly 
preserved  than  that  of  the  Cambrian,  and  appears  to  have  been 
more  varied  and  advanced.  Among  its  simpler  forms  were 
Foraminifera  and  sponges.  A  foraminifer  (of  which  there  were 


2GO 


GEOLOGY 


no  doubt  representatives  in  Cambrian  times,  and  there  are  still 
many  living  types  in  the  present  ocean,  see  Fig.  33)  is  generally 
a 'minute  animal,  composed  of  a  jelly-dike  substance  which, 
possessing  no  definite  organs,  has  in  some  kinds  the  power  of 
secreting  a  hard  calcareous  or  horny  shell,  through  openings  or 
pores  (foramina)  in  which  filaments  from  the  jelly-like  mass 
are  protruded.  By  other  kinds,  grains  of  sand  are  cemented 
together  to  form  a  protecting  shell.  It  is  these  calcareous  and 
sandy  coverings  which  occur  in  the  fossil  state  and  prove  the 
presence  of  foraminifera  in  the  older  oceans  of  the  globe. 
Sponges  also  are  known  to  have  existed  in  the  Cambrian  and 
Silurian  seas,  and  their  remains  have  been  met  with  in  all  parts 
of  the  Geological  Record  down  to  the  present  day.  It  is,  of 


Fig.  121. —  Graptolites  from  Silurian  rocks.  A,  Rastrites  Linncei.  B, 
Monograptus  pnodon.  C,  Diplograptus  pristis,  D,  Puyllograptus  ty- 
pus.  E$  Didymograptus  Murchisonii  (all  natural  size). 

course,  only  where  these  animals  secrete  hard  durable  parts  that 
they  can  be  detected  as  fossils.  A  sponge  is  a  mass  of  soft, 
transparent,  jelly-like  substance,  perforated  by  tubes  or  canals, 
and  supported  on  an  internal  network  of  minute  calcareous  or 
siliceous  spicules,  or  of  interlacing  horny  fibres.  Most  fossil 
sponges  are  calcareous  or  siliceous,  and  their  hard  parts,  being 
durable,  have  been  preserved  sometimes  in  prodigious  numbers 
and  in  wonderful  perfection.  The  common  sponge  of  domestic 
use  is  an  example  of  the  horny  type. 

The  Hydrozoa  were  abundantly  represented  in  the  Silurian 
seas  by  graptolites,  of  which  there  were  many  different  kinds. 
Some  of  the  more  characteristic  of  these  are  shown  in  Fig.  121. 


THE  SILURIAN  PERIOD  261 

They  abound  in  certain  bands  of  shale,  both  in  the  Lower  and 
Upper  Silurian  series,  the  double  forms  (such  as  C,  Fig.  121) 
being  more  characteristic  of  the  Lower  division,  while  the  single 
forms  run  throughout  the  system. 

Corals  abound  in  some  parts  of  the  Silurian  seas.  Their 
remains  chiefly  occur  in  the  limestones,  doubtless  because  these 
rocks  were  formed  in  comparatively  clear  water,  in  which  the 
corals  could  flourish.  But  they  differed  in  structure  from  the 
familiar  reef-building  corals  of  the  present  day.  The  great 
majority  of  them  belonged  to  the  family  of  the  Eugose  corals, 
now  only  sparingly  represented  in  the  waters  of  the  present 
ocean.  As  their  name  denotes,  they  were  particularly  marked 
by  their  thick  rugged  walls.  Many  of  them  were  single  inde- 
pendent individuals;  some  lived  together  in  colonies;  while 


Fig.  122. —  Silurian  Corals,     (a)    Rugose  Coral   (Omphyma  turbinatum, 
y2).     (&)  Alcyonarian  Coral  (Heliolites  inter stinctus,  natural  size). 

others  were  sometimes  solitary,  sometimes  gregarious.  A  typical 
example  of  these  rugose  forms  is  Omphyma,  shown  in  Fig.  122 
(a).  Other  genera  were  Cyathaxonia,  Cyathophyllum,  and 
Zaphrentis.  There  were  likewise  less  numerous  and  more  deli- 
cate compound  forms  belonging  to  what  are  known  as  the 
Tabulate  corals  (Favosites,  Halysites) ,  while  another  type 
(Heliolites,  Fig.  122,  b)  represented  in  ancient  times  the  Alcy- 
onarian corals  (Heliopora)  of  the  present  time. 

Crinoids  or  stone-lilies  played  an  important  part  in  the  earlier 
seas  of  the  globe.  In  some  regions  they  lived  in  such  abun- 
dance on  the  sea-floor  that  their  aggregated  remains  formed 
solid  beds  of  limestone  hundreds  of  feet  thick,  and  covering 
thousands  of  square  miles.  As  their  name  denotes,  crinoids  are 


262  GEOLOGY 

lily-shaped  animals,  having  a  calcareous,  jointed  flexible  stalk 
fixed  to  the  bottom  and  supporting  at  its  upper  end  the  body, 
which  is  composed  of  calcareous  plates  furnished  with  branched 
calcareous  arms  (see  Figs.  149,  165,  173).  It  is  these  hard 
calcareous  parts  which  have  been  so  abundantly  preserved  in 
the  fossil  state.  Eemains  of  crinoids  are  found  in  various  parts 
of  the  Silurian  system  (Dendrocrinus,  Glyptocrinus)  chiefly  in 
the  limestones,  but  not  in  such  abundance  and  variety  as  in 
later  portions  of  the  Palaeozoic  formations  (compare  pp.  274, 
286,  and  Figs.  149,  165,  and  173).  Allied  to  the  crinoids 
were  the  Cystideans,  a  curious  order  of  echinoderms,  with 
rounded  or  oval  bodies  enclosed  in  calcareous  plates,  possessing 
only  rudimentary  arms,  and  a  comparatively  small  and  short 
jointed  stalk.  They  'first  appeared  in  the  Cambrian  period 


Fig.  123. —  Silurian  Echinoderms.      (a)    Cystidean   (Pseudocrinites  quad- 
rifasdatus,  natural  size).    (&)    Star-fish    (Palceasterina  stellata,  3|). 

(Protocystites) ,  but  attained  their  chief  development  during 
Silurian  time,  thereafter  diminishing  in  numbers.  They  are 
thus  characteristically  Silurian  types  of  life.  One  of  them 
is  represented  in  Fig.  123  (a).  Star-fishes  and  brittle-stars 
likewise  occur  as  fossils  among  the  Silurian  rocks.  These 
marine  creatures,  still  represented  in  our  present  seas,  possess 
hard  calcareous  plates  and  spines,  which,  being  imbedded  in 
a  tough  leathery  integument,  have  not  infrequently  been  pre- 
served in  their  natural  position  as  fossils.  Some  of  the  genera 
of  star-fishes  found  in  the  Silurian  system  are  Palceaster, 


THE  SILURIAN  PERIOD  263 

Palceasterina    (Fig.   123,   &),  Pdlceochoma.     Brittle-stars  were 
represented  by  Protaster. 

In  the  Silurian  system  are  found  many  tracks  and  burrows 
like  those  of  the  Cambrian  rocks,  indicative  of  the  presence 
of  different  kinds  of  sea-worms.  Throughout  great  thicknesses 
of  strata,  indeed,  these  markings  are  sometimes  the  only  or 
chief  fossils  to  be  found.  Names  have  been  given  to  the  differ- 
ent kinds  of  burrows  (Arenicolites,  Scolitlius,  Lumbricaria,  Fig. 
124),  and  of  trails  (Palceo chorda,  Palceophycus) .  There  were 
likewise  representatives  of  the 
familiar  Serpula,  which  is 
found  so  abundantly  on  the 
present  sea-bottom,  encrusting 
shells  and  stones  with  a  calca- 
reous protecting  tube,  inside  of 
which  the  annelide  lives.  This 
tube  has  been  preserved  in  the 

fossil  state  in  rocks  of  all  ages.    Fiff>  124.— Filled-up    Burrows    or 

TTiP  TrilnVu'f^c    w"hir>li  "ha^  al          Trails  left  by  a  sea- worm  on  the 
ine   irilObltes,  WHICH  nad.  al-        bed  of  the  Silurian  sea  (Lumbri- 

ready  appeared  in  Cambrian  caria  antiqua,  %) . 
time,  attained  their  maximum  development  during  the  Silurian 
period.  A  few  of  the  primordial  or  Cambrian  types  continued 
to  live  into  this  period,  but  many  new  genera  appeared.  In 
the  Lower  Silurian  series  some  of  the  more  abundant  genera 
are  Asaplius,  Ampyx,  Ogygia,  and  Trinucleus;  in  the  Upper 
Silurian  division  characteristic  genera  are  Calymene,  Phacops, 
Encrinurus,  Illcenus,  and  Homalonotus  (Fig.  125).  Trilobites 
continued  to  nourish,  but  in  gradually  diminishing  variety, 
during  the  Devonian  and  Carboniferous  periods,  after  which 
they  seem  to  have  died  out.  They  are  thus  a  distinctively 
Palaeozoic  type  of  life,  each  great  division  of  the  Paleozoic 
rocks  being  characterised  by  its  own  varieties  of  the  type. 

Phyllopod  crustaceans  likewise  attained  to  greater  variety 
during  the  Silurian  period;  some  of  the  more  frequent  genera 
are  Ceratiocaris  (Fig.  126),  Discinocaris,  and  Caryocaris.  The 
Phyllopods  attained  their  maximum  development  during  Palaeo- 
zoic time,  but  they  have  continued  in  existence  ever  since, 
and  are  at  present  represented  by  a  number  of  genera,  some 
of  which  live  in  the  sea,  others  in  fresh  water. 


264 


GEOLOGY 


The  Mollusca  are  far  more  abundant  and  varied  in  the 
Silurian  than  in  the  Cambrian  rocks.  Among  the  more  char- 
acteristic Silurian  genera  of  Brachiopods  are  Airy  pa,  Leptcena, 
Orthis,  Pentamerus,,  Rhynclionella,  and  Strophomena  (Fig.  127). 
Among  the  lamellibranchs  we  find  the  Cambrian  genera 


Fig.  125. —  Lower  and  Upper  Silurian  Trilobites.  (a)  Asaphus  ty- 
rannus  (J)  ;  (6)  Ogygia  Buchii  (J)  ;  (c)  Illcenus  barriensis  (J)  ; 
(d)  Trinucleus  concentricus  (natural  size)  ;  (e)  Homalonotus  del- 
phinocephalus  (S). 

Ctenodonta  and  Modiolopsis,  with  new  forms  such  as  Orthonota 
(Fig.  128),  CleidopJiorus  and  Amlonychia. 

The  Gasteropods  played  an  important  part  in  the  fauna  of 
the  Silurian  sea,  for  upwards  of  1300  species  of  them  have 


THE  SILURIAN  PERIOD 


265 


been  found  in  Silurian  rocks.  Among  the  more  frequent  genera 
are  Belleroplion  (Fig.  129),  Opliileta,  Holopea,  Murchisonia, 
Platyschisma. 

Numerous  representatives  of  the 
chambered  cephalopods  have  been 
found  in  the  Silurian  rocks,  especially 
in  the  upper  division.  Among  the 
more  frequent  genera  are  Orthoceras 
(straight,  Fig.  130  a),  Cyrtoceras 
(curved),  Ascoceras  (globular  or  pear- 
shaped),  Lituites  (coiled,  Fig.  130  &), 
and  also  Nautilus,  a  genus  which  has 


g.    iztj 

Phyllopod  Crustacean 

( Ceratiocaris        papi- 

lio). 


persisted  through  the  greater  part  of  geological  time  to  the  pres- 
ent day,  and  now  remains  the  only  representative  of  the  cham- 
bered cephalopods  formerly  so  abundant. 


Fig.  127. —  Silurian  Brachiopods.  (a)  Atrypa  reticularis  (natural  size), 
Caradoc  beds  to  Lower  Devonian;  (6)  Orthis  actonice  (natural  size)  ; 
(c)  Rhynchonella  lorealis  (natural  size)  ;  (d)  Pentamerus  galeatus 
(natural  size). 

Remains  of  Fishes  detected  in  the  Upper  Silurian  rocks  are 
the  earliest  traces  of  vertebrate  life  yet  known.  They  consist 
partly  of  plates  which  are  regarded  as  portions  of  the  bony 
covering  of  certain  placoderms  or  bone-plated  forms  (Pteraspis, 


266 


GEOLOGY 


Cephalaspis,  AucJienaspis) ;  partly  of  curved  spines  and  shag- 
reen-like fragments.  The  creatures  of  which  these  are  relics 
appeared  as  forerunners  of  the  remarkable  assemblage  of  fishes 
in  the  next  geological  period  (see  p.  270).  All  the  animal 
remains  hitherto  enumerated  are  relics  of  the  inhabitants  of 
the  sea.  Of  the  land-animals  of  the  time  nothing  was  known 
until  the  year  1884,  when,  by  a  curious  coincidence,  the  dis- 
covery was  made  of  the  remains  of  scorpions  in  the  Silurian 
rocks  of  Sweden,  Scotland,  and  the  United  States,  and  of  an 
insect  allied  to  the  living  cockroach  (Palceoblattina)  in  those 
of  France.  If  scorpions  and  insects  existed  during  this  ancient 
period  we  may  be  sura  that  other  forms  of  terrestrial  life  were 
also  present.  A  new  interest  is  thus  given  to  the  prosecution 
of  the  search  for  fossils  among  the  older  formations. 


Fig.  128. —  Silurian  L  a  m  e  1 1  i- 
branch  (Orthonota  semisulcata 
natural  size). 


Fig.    129. —  Silurian 
Gasteropod     (Bellero 
phon  dilatatus,  &). 


Putting  together  evidence  furnished  by  rocks  and  fossils  of 
the  Silurian  system,  we  get  a  glimpse  of  the  aspect  of  the 
globe  during  the  early  geological  period  which  they  represent. 
The  rocks  bring  before  us  the  sand,  mud,  and  gravel  of  the 
bottom  of  the  sea,  and  tell  of  some  old  land  from  which  these 
materials  were  worn  away.  The  detritus  carried  out  from  the 
shores  of  that  land  was  laid  down  upon  the  sea-bottom  just 
as  similar  materials  are  being  disposed  of  at  the  present  day. 
The  area  occupied  by  Silurian  rocks  marks  out  the  tracts  then 
covered  by  the  sea.  Following  these  upon  a  map  we  perceive 
that  vast  regions  of  the  existing  continents  were  then  parts 
of  the  ocean-floor.  In  Europe,  for  example,  Silurian  rocks  un- 
derlie the  greater  part  of  the  British  Islands,  whence  they 
stretch  northwards  across  a  large  part  of  Scandinavia  and 
the  basin  of  the  Baltic.  They  rise  to  the  surface  in  many  places 


THE  SILURIAN  PERIOD 


2G7 


on  the  continent  from  Spain  to  the  Ural  Mountains.  They 
are  found  forming  parts  of  some  of  the  great  mountain-chains 
of  the  globe,  as,  for  instance,  in  the  Cordilleras  of  South 
America,  in  the  Alps,  and  in  the  Himalayas.  Even  at  the 
antipodes  they  are  met  with  as  thick  masses  in  Australia  and 
New  Zealand.  It  is  evident  that  the  geography  of  the  globe 
in  Silurian  times  was  utterly  unlike  what  it  is  now.  A  large 


Fig.   130. —  Silurian   Cephalopods.      (a)    Orthoceras   emeritum    (S)  ;    (6) 
Trochoceras  (Lituites)  cornu-arietis  (i). 

part  of  the  present  land  was  then  covered  with  shallow  seas, 
in  which  the  Silurian  sedimentary  rocks  were  laid  down.  There 
would  seem  to  have  been  extensive  masses  of  land  in  the  boreal 
part  of  the  northern  hemisphere  connecting  the  European, 
Asiatic,  and  American  continents.  Along  the  coast-line  of  the 
northern  land  and  across  the  shallow  seas  lying  to  the  south 
of  it,  the  same  species  of  marine  organisms  migrated  freely 
between  the  Old  and  the  New  Worlds. 

The  following  Table  shows  the  subdivisions  which  have  been 
made  in  the  Silurian  system  of  Britain. 

Ludlow  group  (mudstone  and  Aymestry  limestone)  — 
Kirkby  Moor  and  Bannisdale  flags  and  slates. 

Wenlock  group  (shales  and  limestones)  —  Denbighshire 
and  Coniston  grits  and  flags. 

Upper  Llandovery  group  —  May  Hill  sandstones. 


s| 

II 


Lower  Llandovery  group  —  grits  and  sandstones. 

Bala  and  Caradoc  group  —  sandstones,  slates,  and  grits, 
with  Bala  (Coniston)  limestone. 

Llandeilo  group  —  dark  argillaceous  and  sometimes  cal- 
careous flagstones  and  shales. 

Arenig  group  —  dark  slates,  flags,  and  sandstones. 


GEOLOGY 


CHAPTER  XIX. 

DEVONIAN  AND  OLD  RED  SANDSTONE. 

THE  DEVONIAN  system,  which  comes  next  in  order,  was 
named  by  Sedgwick  and  Murchison  after  the  county  of 
Devon  where  they  studied  its.  details.  In  Europe,  and 
likewise  in  the  eastern  part  of  North  America,  it  occurs  in  two 
distinct  types,  which  bring  before  us  the  records  of  two  very 
different  conditions  in  geography  of  these  regions  during  the 
time  when  the.  rocks  composing  the  system  were  being  deposited. 
The  ordinary  type,  which  occurs  all  over  the  world,  represents 
the  tracts  that  were  covered  by  the  sea,  and  has  preserved  the 
remains  of  many  forms  of  the  marine  life  of  the  period.  It 
is  that  to  which  the  name  Devonian  is  more  particularly  ap- 
plicable. The  less  frequent  type,  is  characterised  by  thick 
accumulations  of  sandstones,  flagstones,  and  conglomerates  that 
were  laid  down  in  lakes  and  Inland  seas,  and  contain  a  distinct 
assemblage  of  land  and  fresh-water  fossils.  This  lacustrine 
type  is  known  by  the  name  of  OLD  BED  SANDSTONE. 

In  their  general  character  the  Devonian  rocks  resemble  those 
of  the  Silurian  system  underneath.  In  Central  Europe,  where 
they  attain  a  thickness  of  many  thousand  feet,  their  lower 
division  consists  mainly  of  sandstones,  grits,  graywackes,  slates, 
and  phyllites.  The  central  zone  contains  thick  masses  of  lime- 
stone, often  full  of  corals  and  shells,  while  the  upper  portions 
comprise  thin-bedded  sandstones,  shales,  and  limestones.  These 
various  strata  represent  the  sediments  intermittently  laid  down 
upon  .the  bottom  of  the  sea  which  then  covered  the  greater 
part  of  Europe.  Here  and  there,  they  include  bands  of  diabase 
and  tuff,  which  show  that  submarine  volcanic  eruptions  took 
place  during  their  deposition. 

In  the  north-west  of  Europe,  however,  the  floor  of  the  Silurian 
sea  was  irregularly  ridged  up  into  land,  and  large  lakes  were 


DEVONIAN  AND  OLD  RED  SANDSTONE 


2G9 


formed,  into  which  rivers  from  the  ancient  northern  continent 
poured  enormous  quantities  of  gravel,  sand,  and  silt.  The  sites 
of  these  lakes  can  be  traced  in  Scotland,  the  north  of  England, 
and  Ireland.  Similar  evidence  of  land  and  lake-waters  is 
found  in  New  Brunswick  and  Nova  Scotia.  That  some  of  the 
larger  lakes  were  marked  by  lines  of  active  volcanoes  is  well 
shown  in  Central  Scotland,  where  the  piles  of  lava  and  ashes 
left  by  the  eruptions  are  more  than  6000  feet  thick. 

The  occurrence  of  both  marine  and  lacustrine  deposits  is  of 
the  highest  interest,  for,  on  the  one  hand,  we  learn  what  kinds 
of  animals  lived  in  the  sea  in  succession  to  those  that  peopled 
the  Silurian  waters,  and,  on  the  other  hand,  we  meet  with 
the  first  abundant  remains  of  the  vegetation  that  covered  the 


Fig.  131.— Plants  of  the  Devonian  period,     (a)  Psilophyton  (^4)  ;   (I) 
Palceopteris   ( % ) . 

land,  and  of  the  fishes  that  inhabited  the  fresh  waters.  The 
terrestrial  flora  of  the  Devonian  period  has  been  only  sparingly 
preserved  in  the  marine  strata;  but  occasional  drifted  speci- 
mens occur  to  show  that  land  was  not  very  distant  from  the 
tracts  on  which  these  strata  were  laid  down.  In  the  lacustrine 
series  or  Old  Eed  Sandstone  of  Britain  more  abundant  remains 
have  been  met  with,  but  the  chief  sources  of  information  re- 
garding this  flora  are  to  be  sought  in  New  Brunswick  and 
Gaspe,  where  upwards  of  100  species  of  plants  have  been  dis- 
covered. Both  in  Europe  and  in  North  America,  the  Devonian 
vegetation  was  characterised  by  the  predominance  of  ferns, 
lycopods  (Lepidodendron,  etc.),  and  calamites.  It  was  essen- 
tially acrogenous  —  that  is,  it  consisted  mainly  of  flowerless 


270  GEOLOGY 

plants  like  our  modern  ferns,  club-mosses,  and  horse-tail  reeds. 
Traces  of  coniferous  plants,  however,  show  that  on  the  upland 
of  the  time  pine-trees  grew,  the  stems  of  which  were  now  and 
then  swept  down  by  floods  into  the  lakes  or  the  sea. 

While  the  general  aspect  of  the  flora  was  uniformly  green 
and  somewhat  monotonous,  the  fauna  had  now  become  increas- 
ingly varied.  We  know  that  these  early  woodlands  were  not 
without  insect  life,  for  neuropterous  and  orthopterous  wings 
have  been  preserved  in  the  strata.  Some  of  these  remains 
indicate  the  existence  of  ancient  forms  of  ephemera  or  May-fly, 
one  of  which  was  so  large  as  to  have  a  spread  of  wing  measur- 
ing 5  inches  across.  -There  were  likewise  millipedes,  which 
fed  on  the  decayed  wood  of  the  forests.  Traces  of  land-snails 
too  have  been  detected  among  the  fossil  vegetation.  It  is  evi- 
dent, however,  that  the  plant  and  animal  life  of  the  land  has 
only  been  sparingly  preserved;  and  though  our  knowledge  of 
it  has  in  recent  years  been  largely  increased,  we  shall  probably 
never  discover  more  than  a  mere  fragmentary  representation  of 
what  the  original  terrestrial  flora  and  fauna  really  were. 

The  lake-basins  of  the  Old  Red 
Sandstone  have  yielded  large 
numbers  of  remains  of  the 
fishes  of  the  time.  These  are 
members  of  the  remarkable  or- 
der of  Ganoids  —  the  earliest 
known  type  of  fishes  —  which, 
though  so  abundant  in  early 
geological  time,  is  represented 
Fig.  132. —  Overlapping  scales  ^  ^  nrpqpnt  rlav  hv  rvnlv  a 
of  an  Old  Red  Sandstone  ai  aaJ  DJ  'nv  a 

fish    (Holoptychius   Ander-     few  widely  scattered  species,  such 

as   the   sturgeon,  the  polypterus 

of  the  Nile,  and  the  bony  pike  or  garpike  of  the  American  lakes. 
These  modern  forms  are  denizens  of  fresh  water,  and  there 
is  reason  to  believe  that  their  early  ancestors  were  also  in- 
habitants of  lakes  and  rivers,  though  many  of  them  may  also 
have  been  able  to  pass  out  to  the  sea.  The  ganoids  are  so 
named  from  the  enamelled  scales  and  plates  of  bone  in  which 
they  are  encased.  In  some  of  the  fossil  forms,  this  defensive 
armour  consisted  of  accurately  fitting  and  overlapping  scales 
(-Figs.  132,  133) ;  in  others,  the  head  with  more  or  less  of  the 


DEVONIAN  AND  OLD  RED  SANDSTONE 


271 


body  was  protected  by  large  and  thick  plates  of  bone  (Fig.  134). 
Examples  of  both  these  kinds  of  armature  are  to  be  observed 
among  the  fishes  of  the  Old  Eed  Sandstone.  Some  of  the 


Fig.    133. —  Scale-covered    Old    Red    Sandstone    fishes,     (a)     Osteolepis; 
(&)    Acanthodes    (both    reduced). 

most  characteristic  scale-covered  genera  are  Osteolepis,  Diplop- 
terus,  Glyptolcemus,  Holoptychius,  Acanthodes  (Figs.  132,  133). 


Fig.   134. —  Plate-covered   Old  Red   Sandstone   fishes,     (a)    Cephalaspis; 
(6)    Pterichthys    (both    reduced). 

The  acanthodians  (Fig.  133,  &),  distinguished  by  the  thorn- 
like  spines  supporting  their  fins,  reached  their  greatest  develop- 
ment during  the  Devonian  period.  Of  the  plate-covered  ganoids 
or  placoderms  some  of  the  most  characteristic  were  the  curious 
Cephalaspis  (Fig.  134,  a),  with  its  head-buckler  shaped  like  a 
saddler's  awl,  the  Pteraspis,  which,  with  Cephalaspis.,  had  al- 
ready appeared  in  the  Silurian  period,  the  Coccosteus  and 
Pterichthys  (Fig.  134,  I).  Some  of  the  contemporaries  of 


272  GEOLOGY 

these  creatures  attained  a  great  size.  Thus  the  Asterolepis  had 
its  head  and  shoulders  encased  in  a  buckler,  which  in  some 
examples  is  20  inches  long  by  16  broad.  Still  larger  were  some 
of  its  American  allies,  one  of  which,  the  Dinichthys,  had  a 
head-buckler  3  feet  long  armed  with  formidable  teeth. 

One  of  the  fishes  of  the  Old  Eed  Sandstone,  named  Dipterus, 
has  recently  been  found  to  have  a  singular  modern  representa- 
tive in  the  barramunda  or  mud-fish  (Ceratodus)  of  the  Queens- 
land rivers  in  Australia.  Dipterus  resembled  the  ganoids  in  its 
external  enamel  and  strong  bony  helmet,  but  its  jaws  present 
the  characteristic  teeth,  and  its  scales  have  the  rounded  or 
"cycloid"  form  of  'Ceratodus.  That  some  of  these  fishes 
swarmed  in  the  waters  of  the  Old  Eed  Sandstone  is  shown  by 
the  prodigious  numbers  of  their  remains  occasionally  preserved 
in  the  sandstones  and  flagstones.  Their  bodies  lie  piled  on 
each  other  in  such  numbers,  and  often  so  well  preserved,  as  to 
show  that  probably  the  animals  were 
suddenly  killed,  and  were  covered  up 
with  sediment  before  their  remains  had 
time  to  decay  and  to  be  dispersed  by  the 
currents  of  water.  Perhaps  earth- 
quake shocks,  or  the  copious  discharge 
of  mephitic  gases,  or  other  sudden  bane- 
ful influence,  may  have  been  the  cause 
of  the  extensive  destruction  of  life  in 
these  ancient  waters. 

That  some  of  the  fishes  found  their 

Fig.  135.— Devonian  •  wa?  to  the  sea>  as  our  modern  salmon 
Eurypterid  Crustacean  does,  is  indicated  by  the  occasional  oc- 
(Pterygotus,  reduced).  currence  of  their  remains  among  those 

of  the  truly  marine  fauna  of  the  Devonian  rocks.  But  the  rarity 
of  their  presence  there,  compared  with  their  prodigious  abun- 
dance in  some  parts  of  the  Old  Eed  Sandstone,  probably  serves 
to  show  that  they  were  essentially  inhabitants  of  the  lakes  and 
rivers  of  the  land. 

Among  the  animals  that  appear  to  have  been  migratory 
between  the  sea  and  the  terrestrial  waters,  were  the  curious 
forms  known  as  Eurypterids,  which,  though  generally  classed 
with  the  crustaceans,  had  many  affinities  with  the  arachnids 
or  scorpions.  One  of  the  most  remarkable  of  these  creatures 


DEVONIAN  AND  OLD  RED  SANDSTONE 


273 


was  faePterygotus,  of  which  the  general  form  is  shown  in  Fig. 
135.  Most  of  the  species  are  small,  though  one  of  them  found 
in  Scotland  must  have  attained  a  length  of  5  or  6  feet. 

But  it  is  the  marine  or  Devonian  fauna  which  is  most  widely 
spread  over  the  globe,  and  from  its  extensive  distribution  is 


136. —  Devonian  Trilobites.  (a)  Bronteus  flalellifer  (J)  ;  (6) 
Dalmanites  rugosa  (S)  ;  (c)  Homalonotus  armatus  (4)  ;  (d)  Harpes 
macrocephalus  (|). 

of  most  importance  to  the  geologist.  Taken  as  a  whple,  it 
presents  a  general  resemblance  to  that  of  the  Silurian  period 
which  it  succeeded.  Some  of  the  Silurian  species  survived  in 


Fig.     137. —  Devonian    Corals,      (a)    Cyathophyllum   ceratites    (J)  ;   (6) 
Calceola  sandalina  (§). 

it,  and  new  species  of  the  old  genera  made  their  appearance. 
But  important  differences  are  to  be  observed  between  the  faunas 
of  the  two  systems,  showing  the  long  lapse  of  time,  and  the 
changes  which  it  brought  about  in  the  life  of  the  globe. 

It  is  specially  interesting  to  mark  how  some  of  the  char- 


274 


GEOLOGY 


acteristic  Silurian  types  dwindle  and  finally  die  out  in  the 
Devonian  system.  One  of  the  best  examples  of  this  survival 
and  disappearance  is  supplied  by  the  Graptolites.  It  will  bo 
remembered  how  prodigiously  abundant  these  creatures  were 
in  the  Silurian  seas.  They  are  met  with  also  in  scattered 
specimens  in  the  lower  and  middle  divisions  of  the  Devonian 
system,  but  their  rarity  there  affords  a  striking  contrast  to 
their  profusion  among  the  Silurian  strata,  and  they  seem  to 
have  entirely  died  out  before  the  end  of  the  Devonian  period, 
for  no  traces  of  them  occur  in  the  later  parts  of  the  system, 


Fig.    138. —  Devonian    Brachiopods.      (a)     Uncites    gryphus     (§)  ;     (6) 
String ocephalus  Burtini  (§)  ;  (c)  Spirifera  disjuncta  (Verneuillii)   (i). 

and  they  have  never  been  met  with  in   any  later  geological 
formation. 

Again,  Trilobites,  which  form  such  a  predominant  and  strik- 
ing feature  of  the  Silurian  fauna,  occur  in  greatly  diminished 
number  and  variety  among  the  Devonian  rocks.  Most  of  the 
Silurian  genera  are  absent,  some  of  the  most  frequent  Devonian 
types  being  PTiacops,  Cryphceus,  Homalonotus,  Dalmanites,  and 
Bronteus  (Fig.  136).  We  shall  find  that  this  peculiarly  Pateo- 


DEVONIAN  AND  OLD  RED  SANDSTONE       275 

zoic  type  of  Crustacea  finally  died  out  in  the  next  or  Carbon- 
iferous period.  But  while  the  trilobites  were  waning,  the  euryp- 
terids,  already  referred  to,  appeared  and  attained  a  great 
development. 

In  the  clearer  parts  of  the  sea  vast  numbers  of  rugose  corals 
flourished,  and,  with  other  calcareous  organisms,  built  up  solid 
masses  of  limestone.  Some  of  the  characteristic  genera  were 
CyatJiopliyllum  (Fig.  137),  Acervularia,  Cystiphyttum,  and  the 
curious  Calceola  which,  after  being  successively  placed  among 
the  lamellibranchs  and  the  brachiopods,  is  now  regarded  as  a 
rugose  coral  with  an  opercular  lid.  With  these  were  likewise 
associated  vast  numbers  of  crinoids,  of  which  the  genera 
Cyathocrinus  and  Cupressocrinus  were  especially  characteristic. 

The  Brachiopods  reached  their  maximum  of  development 
in  the  Devonian  seas,  upwards  of  60  genera  and  1100  species 


Fig.    139. —  A,    Devonian    Lamellibranch    (Cucullcea    Hardingii,    §).     B, 
Devonian  Cephalopod  (Clymenia  Sedgwickii,  §). 

having  been  described  from  Devonian  rocks.  Comparing  them 
with  those  of  the  Silurian  system,  we  notice  that  some  of  the 
most  characteristic  Silurian  types,  such  as  forms  of  Orthis  and 
Strophomena,  became  fewer  in  number,  while  forms  of  Pro- 
ductus  and  Chonetes  increased.  The  most  abundant  families 
were  those  of  the  Spirifers  (Uncites,  Cyrtia,  Athyris,  Atrypa) 
and  Ehynchonellids  (Fig.  138).  Two  distinctively  Devonian 
brachiopods  were  StringocepJialus  and  Rensseleria,  allied  to  the 
still  living  Terebratula.  The  former  is  especially  characteristic 
of  one  of  the  Middle  limestones. 

The  other  mollusca  appear  to  have  been  well  represented  in 
the  Devonian  seas.  Of  the  Lamellibranchs,  Pterinea  is  par- 
ticularly abundant  in  the  lower  part  of  the  system,  Cucullcea 


276 


GEOLOGY 


Fig.  139,  A)  in  the  upper  part.  The  Devonian  cephalopods  in- 
cluded many  species  of  the  genera  Orthoceras,  Cyrtoceras, 
Clymenia,  Goniatites,  and  Bactntes  (Fig.  139,  B). 

The  Devonian  system  in  Europe  is  subdivided  as  in  the  sub- 
joined Table : 


Upper 


Lower 


Middle   < 


Pilton  and  Pickwell-Down  group  of  England  —  Upper 
Old  Red  Sandstone  of  Scotland. 

Famennian  and  Frasnian  sandstones,  shales,  and  lime- 
stones of  the  North  of  France  and  Belgium  —  Psani- 
mites  de  Condroz. 

Cypridina-shales,  Spirifer  sandstone,  Rhynchonella 
culoides  beds  of  Germany. 

Ilfracombe  and  Plymouth  limestones,  grits,  and  con- 
glomerates of  Devonshire.  [No  middle  Old  Red  Sand- 
stone.] 

Limestone  of  Givet,  and  Calceola  shales  of  North  of 
France. 

Stringocephalus-limestone  of  the  Eifel  —  Calceola-group 
of  Germany. 

Linton  slates  and  sandstones  of  Devon  and  Cornwall  — 
Lower  Old  Red  Sandstone  of  Scotland  and  Wales. 

Coblenzian,  Taunusian,  and  Gedinnian  rocks  of  the  Ar- 
dennes and  Taunus. 


THE  CARBONIFEROUS  PERIOD  277 


CHAPTER  XX. 

THE    CARBONIFEROUS    PERIOD. 

THE  next  great  division  of  the  Geological  Eecord  has  re- 
ceived the  name  of  CARBONIFEROUS,  from  the  beds  of 
coal  (Latin  Carlo)  which  form  one  of  its  most  conspicu- 
ous features.  The  rocks  of  which  it  consists  reach  sometimes  a 
thickness  of  fully  20,000  feet,  and  contain  the  chronicle  of  a 
remarkable  series  of  geographical  changes  which  succeeded  the 
Devonian  period.  They  include  limestones  made  up  in  great 
part  of  corals,  crinoids,  polyzoa,  brachiopods,  and  other  calcare- 
ous organisms  which  swarmed  in  the  clearer  parts  of  the  sea ; 
sandstones  often  full  of  coaly  streaks-  and  remains  of  terrestrial 
plants;  dark  shales  not  infrequently  charged  with  vegetation, 
and  containing  nodules  and  seams  of  clay-ironstone;  and  seams 
of  coal  varying  from  less  than  an  inch  to  several  feet  or  yards 
in  thickness,  and  generally  resting  on  beds  of  fire-clay. 

These  various  strata  are  disposed  in  such  a  way  as  to  afford 
clear  evidence  of  the  physical  geography  of  large  areas  of  the 
earth's  surface  during  .the  Carboniferous  period.  The  lime- 
stones attain  a  thickness  of  sometimes  several  thousand  feet, 
with  hardly  any  intermixture  of  sedimentary  material.  They 
consist  partly  of  aggregated  masses  of  corals  and  coralloid 
animals,  which  grew  on  the  sea-floor  somewhat  after  the  man- 
ner of  modern  coral-reefs ;  partly  of  aggregated  stems  and  joints 
of  crinoids,  which  must  have  flourished  in  prodigious  numbers 
on  the  bottom,  mixed  with  fragments  of  other  organisms,  the 
whole  being  aggregated  into  sheets  of  solid  stone.  The  Car- 
boniferous or  Mountain  Limestone,  which  forms  the  lower  part 
of  the  Carboniferous  system,  stretches  from  the  west  of  Ireland 
eastwards  for  a  distance  of  750  miles,  across  England,  Wales, 
Belgium,  and  Rhineland  into  Westphalia.  In  the  basin  of 
the  Meuse  it  is  not  less  than  2500  feet  thick,  and  in  Lancashire, 
where  it  attains  its  maximum  development,  it  exceeds  6000 


278  GEOLOGY 

feet.  Such  an  enormous  accumulation  of  organic  remains 
shows  that,  during  the  time  of  its  deposition,  a  wide  and  clear 
sea  extended  over  the  centre  of  Europe.  But  as  the  limestone 
is  traced  northwards,  it  is  found  to  diminish  in  thickness. 
Beds  of  sandstone,  shale,  and  coal  begin  to  make  their  appear- 
ance in  it,  and  rapidly  increase  in  importance,  as  they  are 
followed  away  from  the  chief  limestone  area;  while  the  lime- 
stone itself  is  at  last  reduced  in  Scotland  to  a  few  beds,  each 
only  a  yard  or  two  in  thickness.  From  this  change  in  the 
character  of  the  rocks,  the  inference  may  be  drawn  that,  while 
the  sea  extended  from  the  west  of  Ireland  eastwards  into 
Westphalia,  land  lyin£  to  the  north  supplied  sand,  mud,  and 
drifted  plants,  which,  being  scattered  over  the  sea-floor,  pre- 
vented the  thick  limestone  from  extending  northwards.  These 
detrital  materials  now  form  the  masses  of  sandstone  and  shale 
that  take  the  place  of  the  limestone  in  the  north  of  England 
and  in  Scotland.  The  northward  extension  of  a  few  limestone 
beds  full  of  marine  organisms  serves  to  mark  a  time  when,  for 
a  longer  or  shorter  interval,  the  water  cleared,  sand  and  mud 
ceased  to  be  carried  so  far  southward,  and  the  corals,  crinoids, 
and  other  limestone-building  creatures  were  able  to  spread  them- 
selves farther  over  the  sea-floor.  But  the  thinness,  of  such 
intercalated  limestones  also  indicates  that  the  intervals  favour- 
able for  their  formation  were  comparatively  short,  the  sandy 
and  muddy  silt  being  once  again  borne  southward  from  the 
land,  killing  off  or  driving  away  the  limestone-builders  and 
spreading  new  sheets  of  sand  and  mud  over  the  site. 

There  can  be  no  doubt  that,  while  these  changes  were  in 
progress,  the  whole  wide  area  of  deposition  in  Western  and 
Central  Europe  was  undergoing  a  gradual  depression.  The 
sea-bottom  was  sinking,  but  so  slowly  that  the  growth  of  lime- 
stone and  the  deposit  of  sediment  probably  on  the  whole  kept 
pace  with  it.  The  actual  depth  of  the  water  may  not  have  varied 
greatly  even  during  a  subsidence  of  several  thousand  feet.  That 
this  was  the  case  may  be  inferred  from  the  structure  of  the 
limestone  itself.  We  have  seen  that  this  rock  sometimes  ex- 
ceeds 6000  feet  in  thickness.  Had  there  been  no  subsidence  of 
the  sea-floor  during  the  accumulation  of  so  thick  a  mass  of 
organic  debris,  it  is  evident  that  the  first  beds  of  limestone 
must  have  been  begun  at  a  depth  of  at  least  6000  feet  below  the 


THE  CARBONIFEROUS  PERIOD  279 

surface  of  Ithe  sea,  and  that,  by  the  gradual  increase  of  cal- 
careous matter,  the  sea  was  eventually  filled  up  to  that  amount, 
f  it  was  not  filled  up  entirely.  But  we  can  hardly  suppose 
that  the  same  kinds  of  organisms  could  live  at  a  depth  of  6000 
feet  and  also  at  or  near  the  surface.  We  should  expect  to  find 
the  organic  contents  of  the  lower  parts  of  the  limestone  entirely 
different  from  those  in  the  upper  parts.  But  though  there  are 
differences  sufficient  to  admit  of  the  limestone  being  separated 
into  stages,  each  marked  by  its  own  distinctive  assemblage  of 
fossils,  the  general  character  or  facies  of  the  organisms  remains 
so  uniform  and  persistent  throughout,  as  to  make  it  quite  certain 
that  the  conditions  under  which  the  creatures  lived  on  the 
bottom  and  built  up  the  limestone  continued  with  but  little 
change  during  the  whole  time  when  the  6000  feet  of  rock  were 
being  deposited.  As  this  could  not  have  been  the  case  had 
there  been  a  gulf  of  6000  feet  to  fill  up,  we  are  led  to  conclude 
that  the  bottom  slowly  subsided  until  its  original  level,  on  which 
the  limestone  began  to  form,  had  sunk  at  least  6000  feet. 

This  conclusion  is  borne  out  by  many  other  considerations. 
Thus  the  sedimentary  strata  that  replace  the  limestone  on  its 
northern  margin  are  also  several  thousand  feet  thick.  But  from 
bottom  to  top  they  abound  with  evidence  of  shallow-water  con- 
ditions of  deposition.  Their  repeated  alternations  of  sandstone, 
grit  (even  conglomerate),  and  shale;  the  presence  in  them  of 
constant  current-bedding;  the  frequent  occurrence  of  ripple- 
marked  and  sun-cracked  surfaces;  the  preservation  of  abundant 
remains  of  terrestrial  vegetation  —  some  of  it  evidently  in  its 
position  of  growth  —  prove  that  the  mass  of  sediment  was  not 
laid  down  in  a  deep  hollow  of  the  sea-bottom,  but  in  shallow 
waters  not  far  from  the  margin  of  the  land. 

But  probably  the  most  interesting  evidence  of  long-continued 
subsidence  during  the  Carboniferous  period  is  furnished  by  the 
history  of  the  coal-seams.  Coal  is  composed  of  compressed  and 
mineralised  vegetation.  In  Britain  each  layer  of  coal  is  usually 
underlain  by  a  bed  of  fire-clay,  or  at  least  of  shale,  through 
which  roots  and  rootlets,  descending  from  the  under  surface 
of  the  coal-seam,  branch  freely.  There  can  be  little  doubt 
that  each  bed  of  fire-clay  is  an  old  soil,  while  the  coal  lying 
upon  it  represents  the  matted  growth  of  vegetation  which  that 


280 


GEOLOGY 


soil  supported.    Hence  the  association  of  a  fire-clay  and  a  coal- 
seam   furnishes  distinct  evidence  of  a  terrestrial  surface.1 

In  many  regions  the  Carboniferous  system  comprises  a  series 
of  sandstones,  shales,  and  other  strata,  many  thousands  of  feet 
in  thickness,  throughout  which,  on  suc- 
cessive platforms,  there  lie  -hundreds  of 
seams  of  coal.  If  each  of  these  seams 
marks  a  former  surface  of  terrestrial 
vegetation,  how  is  this  succession  of 
buried  land-surfaces  to  be  accounted 
for?  There  is  obviously  but  one  so- 
lution of  the  problem.  The  area  over 
which  the  coal-seams  extend  must  have 
been  slowly  sinking.  During  this 
subsidence,  sand,  mud,  and  silt  were 
transported  from  the  neighbouring 
land,  and  in  such  quantity  as  to  fill  up 
the  shallow  waters.  On  the  muddy  flats 
thus  formed,  the  vegetation  of  the  flat 
marshy  swamps  spread  seaward.  There 
may  not  improbably  have  been  pauses 
in  the  downward  movement,  during 
which  the  maritime  jungles  and  forests 
continued  to  flourish  and  to  form  a 
thick  matted  mass  of  vegetable  matter. 
When  the  subsidence  recommenced, 
this  mass  of  living  and  dead  vegetation 
was  carried  down  beneath  the  water 
and  buried  under  fresh  deposits  of  sand 
and  mud.  As  the  weight  of  sediment 
increased,  the  vegetable  matter  would 
be  gradually  compressed  and  would 
slowly  pass  into  coal.  But  eventually 
another  interval  of  rest  or  of  slower 
subsidence  would  allow  the  shallow  sea  once  more  to  be  silted 
up.  Again  the  marsh-loving  plants  from  the  neighbouring 
swampy  shores  would  creep  outward  and  cover  the  tract  with 

^  In  some  Continental  coal-fields  there  is  evidence  that  coal  has  like- 
wise been  formed  out  of  matted  vegetation  which  has  been  swept  down  by 
floods  and  buried  under  sand,  gravel,  and  other  sediment. 


Fig.  140. —  Section  of 
part  of  the  Cape  Bre- 
ton coal-field,  showing 
a  succession  of  buried 
trees  and  land-sur- 
faces, (a)  sandstones; 
(6)  shales;  (c)  coal- 
seams;  (d)  under- 
clays  or  soils. 


THE  CARBONIFEROUS  PERIOD  281 

a  new  mantle  of  vegetation,  which,  on  the  renewal  of  the  down- 
ward movement,  would  be  submerged  and  buried. 

In  the  successive  strata  of  a  coal-field,  therefore,  we  are  pre- 
sented with  the  records  of  a  prolonged  period  of  subsidence, 
probably  marked  by  longer  or  shorter  intervals  of  rest. 
These  more  stationary  periods  are  indicated  by  the  coal- 
seams,  and  perhaps  their  relative  duration  may  be  inferred 
from  the  thickness  of  the  coal.  A  thick  coal-bed  not 
improbably  marks  a  time  of  rest,  when  the  vegetation  was 
allowed  to  flourish  unchecked,  or  when  at  least  the  sinking 
was  so  imperceptible  that  the  successive  generations  of  plants2 
springing  up  on  the  remains  of  their  predecessors,  contrived 
to  keep  themselves  above  the  level  of  the  water. 

In  the  present  world  there  is  no  vegetable  growth  now  in 
progress  quite  like  that  of  the  coal-seams  of  the  Carboniferous 
period.  Perhaps  the  nearest  analogy  is  supplied  by  the  man- 
grove-swamps of  certain  tropical  coasts.  In  these  tracts,  the 
mangrove  trees  grow  seaward,  dropping  their  roots  and  radicles 
into  the  shallow  waters,  and  gradually  forming  a  belt  of  swampy 
jungle  several  miles  broad.  That  the  coal-jungles  extended 
into  the  sea  is  shown  by  the  occurrence  of  marine  shells  and 
other  organisms  in  the  coal  itself.  But  there  were  probably 
also  wide  swamps  wherein  the  water  was  fresh.  A  single  coal- 
seam  may  sometimes  be  traced  over  an  area  of  more  than  1000 
square  miles,  showing  how  widespread  and  uniform  were  the 
conditions  in  which  it  was  formed. 

During  the  subterranean  movements  that  marked  the  Car- 
boniferous period,  the  Devonian  physical  geography  was  en- 
tirely remodelled.  The  lake-basins  of  the  Old  Eed  Sandstone 
were  effaced,  and  the  sea  of  the  Carboniferous  limestone  spread 
over  their  site.  Much  of  the  Devonian  marine  area  was  up- 
ridged  into  land,  and  the  rocks  eventually  underwent  that 
intense  compression  and  plication  which  have  given  them  their 
cleaved,  crumpled,  and  metamorphic  aspect,  and  in  connec- 
tion with  which  they  were  invaded  by  granite  and  intersected 
with  mineral  veins.  It  is  deserving  of  remark  that  volcanic 
action,  which  played  so  notable  a  part  in  Devonian  time,  was 
continued,  but  with  diminished  vigour,  in  the  Carboniferous 
period.  During  the  earlier  half  of  the  period,  volcanic  out- 
bursts were  frequent  in  different  parts  of  Britain,  particularly 


282 


GEOLOGY 


in  Derbyshire,  the  Isle  of  Man,  central  and  southern  Scotland, 
and  the  south-west  of  Ireland.  The  lava  and  ashes  ejected  in 
some  of  these  areas  during  the  time  of  the  Carboniferous 
Limestone  form  conspicuous  groups  of  hills. 

Of  the  plant  and  animal  life  of  the  Carboniferous  period 
much  is  now  known  from  the  abundant  remains  which  have 
been  preserved  of  the  terrestrial  surfaces  and  sea-floors  of  the 
time.  Beginning  with  the  flora,  we  have  first  to  notice  its  gen- 
eral resemblance  to  that  of  the  Devonian  period.  Many  of 
the  genera  of  the  older  time  survived  in  the  Carboniferous 
jungles;  but  other  forms  appear  in  vast  profusion,  which  have 
not  been  met  with  in  any  Devonian  or  Old  Eed  Sandstone  strata. 
The  Carboniferous  flora,  like  that  which  preceded  it,  must  have 


Fig.    141. —  Carboniferous    Ferns. 
(6)    Sphenopteris  artemisice folia 
lonchitica  (4). 


a) 


Neuropteris    macrophylla    (i)  ; 
(c)    Alcthopteris    (Pecopteris) 


been  singularly  monotonous,  consisting  as  it  did  almost  entirely 
of  flowerless  plants.  Not  only  so,  but  the  very  same  species  and 
genera  appear  to  have  then  ranged  over  the  whole  world,  for 
their  remains  are  found  in  Carboniferous  strata  from  the 
Equator  to  the  Arctic  Circle.  Ferns,  lycopods,  and  equisetaceae, 
constituted  the  main  mass  of  the  vegetation.  The  ferns  recall 
not  a  few  of  their  modern  allies,  some  of  the  more  abundant 
kinds  being  Sphenopieris,  Neuropteris,  and  Pecopteris  (Fig. 
141).  Among  the  lycopods  the  most  common  genus  is  Lepido- 
dendron,  so  named  from  the  scale-like  leaf-scars  that  wind  round 
its  stem  (Fig.  1,42).  Its  smaller  branches,  closely  covered  with 
small  pointed  leaves,  and  bearing  at  their  ends  little  cones  or 


THE  CARBONIFEROUS  PERIOD 


283 


spikes  (Lepidostrobus) ,  remind  one  of  the  club-mosses  of  our 
moors  and  mountains;  but  instead  of  being  low-growing  or 
creeping  plants,  like  their  modern  representatives,  they  shot 
up  into  trees,  sometimes  50  feet  or  more  in  height.  Equisetaceaj 
abounded  in  the  Carboniferous  swamps,  the  most  frequent  genus 
being  Calamites,  the  jointed  and  finely-ribbed  stems  of  which 
are  frequent  fossils  in  the  sandstones  and  shales  (Fig.  143,  a). 
This  plant  probably  grew  in  dense  thickets  in  the  sandy  and 
muddy  lagoons,  and  bore  as  its  foliage  slim  branches,  with 
whorls  of  pointed  leaves  set  round  the  joints  (AsteropJiyllites., 
Fig.  143,  &).  The  Sigillarioids  were  among  the  most  abundant, 
and  at  the  same  time,  most  puzzling  members 
of  the  Carboniferous  flora.  They  do  not 
appear  to  have  any  close  modern  allies,  and 
their  place  in  the  botanical  scale  has  been  a 
subject  of  much  controversy.  The  stem  of 
these  trees,  sometimes  reaching  a  height  of  50 
feet  or  more,  was  fluted,  each  of  the  parallel 
ribs  being  marked  by  a  row  of  leaf-scars, 
hence  the  name  Sigillaria.,  from  the  seal-like 
impressions  of  the  scars  (Fig.  144).  These 
surface-markings  disappeared  as  the  tree 
grew,  and  in  the  lower  part  of  the  trunk  they 
passed  down  into  the  pitted  and  tubercular 
surface  characteristic  of  the  roots  (Stig- 
maria),  still  so  abundant  in  their  position  of 
growth  in  fire-clay,  and  also  as  drifted 
broken  specimens  in  sandstones  and  shales. 
Another  plant  that  took  a  prominent  part  in 
the  Carboniferous  flora  was  that  named  Cor- 
daites  (Fig.  145).  Its  true  botanical  place 
is  still  matter  of  dispute;  some  writers  placing  it  with 
the  lycopods,  others  with  the  cycads,  or  even  among  the  conifers. 
It  bore  parallel-veined  leaves  somewhat  like  those  of  a  yucca, 
which,  when  they  fell  off,  left  prominent  scars  on  the  stem, 
and  it  also  carried  spikes  or  buds  (Carpolithes,  Fig.  145).  All 
the  plants  now  enumerated  probably  grew  on  the  lower  grounds 
and  swamps.  But  on  the  higher  and  drier  tracts  of  the  in- 
terior there  grew  araucarian  pines  (Dadoxylon,  Araucarioxylon), 
the  trunks  of  which,  swept  down  by  floods,  were  imbedded  in 


(  Lepidode  n  d  r  o  n 
Sternbergii,  J). 


284 


GEOLOGY 


some  of  the  sands  of  the  time  and  now  appear  petrified  in  the 
sandstones. 
While  the  terrestrial  vegetation  of  the  Carboniferous  period1 


Fig.  143. —  Carboniferous  Equisetaceous  Plants,      (a)   Calamites  Lindleyl 
(  —  C.  Mougeoti,  Lindl,  |)  ;  (6)  Asterophyllites  densifolius  (J). 

has  been  so  abundantly  entombed,  the  fauna  of  the  land  has 
been  but  scantily  preserved.  That  air-breathers  existed,  how- 
ever, has  been  made  known  by  the  finding  of  specimens  of  scor- 


Fig.  144. —  Sigillaria  with  Stigmaria  roots  (much  reduced). 

pions,  myriapods,  true  insects,  and  amphibians.  Within  the 
last  few  years  vast  numbers  of  the  remains  of  scorpions  have 
been  discovered  in  the  Carboniferous  rocks  of  Scotland.  These 


THE  CARBONIFEROUS  PERIOD  285 

ancient  forms  (Eoscorpius)  presented  a  remarkably  close  re- 
semblance to  the  living  scorpion,,  and  so  well  have  they  been 
preserved  among  the  shales  that  even  the  minutest  parts  of 
their  structure  can  be  recognised.  They  possessed  stings  like 
their  modern  descendants,  whence  we  may  infer  the  presence 
of  other  forms  of  life  which  they  killed.  The  Carboniferous 
woodlands  had  plant-eating  millipedes,  and  their  silence  was 
broken  by  the  hum  of  insect-life ;  for  ancestral  forms  of  dragon- 
flies  (Libellulce) ,  May-flies  (Ephemeridce),  stone-flies  (  Perlidce), 
white-ants  (Termidce),  cockroaches  (Blattida),  spectre-insects 
(Phasmidce) ,  crickets  (Gryttidce),  locusts  (Acrydiidce) ,  and 
curious  transitional  forms  between  modern  types  that  are  quite 
distinct  have  been  detected,  chiefly  among  the  shales  and  coals 
3f  the  Coal-measures.  Some  of  these  insects  attained  a  great 
size;  a  single  wing  of  one  of  them  (Megaptilus)  must  have 


Fig.  145. —  Cordaites  alloidius  (|),  with  Carpolithes  attached. 

measured  between  7  and  8  inches  in  length.  While  detached 
wings  and  more  or  less  complete  bodies  have  been  found  as 
rare  and  precious  discoveries  in  many  coal-fields  in  Europe 
and  America,  it  is  at  Commentry  in  France  that  remains  of 
insects  have  been  met  with  in  largest  numbers  —  no  fewer  than 
1300  specimens  having  there  been  disinterred,  most  of  them 
admirably  preserved.  In  the  interior  of  decaying  trees  early 
forms  of  land-snails  lived,  having  a  striking  resemblance  to 
some  kinds  that  are  still  to  be  found  in  our  present  woodlands 
(Pupa) . 

The  lagoons  in  which  the  coal-growths  flourished  were  ten- 
anted by  numerous  forms  of  animal  life.  Among  these  were 
various  mussel-like  molluscs  (Antliracomya,,  Fig.  154,  Anthra- 


286  GEOLOGY 

cosia),  which  were  possibly  restricted  to  fresh  water.  But  wher- 
ever the  sea-water  penetrated,  it  carried  some  of  its  character- 
istic life  with  it,  particularly  Lingula,  Discina,  Aviculopecten, 
Goniatites,  and  other  marine  shells.  The  fishes  of  the  lagoons 
were  chiefly  ganoids  (Hegalichthys,  Rhizodus,  Fig.  158,  Cheiro- 
dus,  Strepsodus,  etc.).  But  some  of  the  rays  and  sharks  from 
the  sea  made  their  way  into  these  waters,  for  their  spines  are 
occasionally  found  among  the  coal-seams  and  shales  (Oyracan- 
thus,  Pleuracanthus,  Fig.  158).  That  the  larger  fishes  lived 
upon  the  smaller  ones  is  shown  by  a  curious  and  interesting 
piece  of  evidence.  Many  of  the  shales  are  full  of  small  oblong 
bodies  which  contain  in  their  interior  the  broken  and  undi- 
gested scales  and  bones  of  small  fishes.  From  their  contents, 
their  peculiar  external  form  and  markings,  and  their  phos- 
phatic  composition,  these  bodies  (coprolites)  are  recognised 
as  the  excrement  of  some  of  the  larger  fishes,  and  the  teeth 
and  scales  within  them  serve  to  show  what  were  the  smaller 
forms  on  which  these  fishes  fed  (see  Fig.  65). 

During  the  Carboniferous  period,  and  indeed  throughout  the 
later  parts  of  the  Paleozoic  ages,  the  most  highly  organised 
creatures  living  on  the  globe,  so  far  as  we  at  present  know, 
belonged  to  the  Amphibia  —  the   great  class  which   includes 
our  modern  frogs,  toads,  and  salamanders. 
They  belonged,  however,  to  an  order  that  has 
long  been  entirely  extinct  —  the  Labyrintho- 
donts,  so  named  from  the  labyrinthine  folds 
^erous~Foramini~   °f  "the  internal  substance  of  their  teeth.    They 
fer  (FusuUna  cy-  were  somewhat  like  the  existing  salamander 

hndrica,  f) .  »  .,,  ,     ,.     ,  .    ., 

in  form,  with  weak  limbs  and  a  long  tail. 
Their  skulls  were  encased  in  strong  plates  of  bone,  and 
they  likewise  carried  protective  bony  scutes  on  the  under  sides 
of  their  bodies.  Those  found  in  Carboniferous  rocks  are  mostly 
small  in  size,  but  some  of  them,  measuring  perhaps  7  or  8  feet 
in  length,  must  have  been  the  monsters  of  the  lagoons  in 
which  they  lived.  Some  of  the  leading  genera  are  Archego- 
saurus,  Anthracosaurus,  Loxomma,  Dendrerpeton,  Baphetes. 

The  marine  life  of  the  Carboniferous  period  has  been  ex- 
tensively preserved  in  the  Carboniferous  Limestone,  which, 
as  already  stated,  consists  of  little  else  than  aggregated  remains 
of  organisms.  In  walking  over  the  surface  of  the  beds  of  this 


THE  CARBONIFEROUS  PERIOD  287 

limestone,  one  treads  upon  the  floor  of  the  sea  in  Carboniferous 
times,  with  its  corals,  crinoids,  and  shells  crowded  and  crushed 
upon  each  other.  Beginning  with  the  most  lowly  of  these 
organisms,  we  may  observe  abundant  remains  of  foraminifera, 
which  in  some  portions  of  the  limestone  constitute  the  greater 
part  of  the  rock.  One  of  their  most  characteristic  forms,  named 
Fusulina  (Fig.  146),  enters  largely  into  the  structure  of  the 
limestone  across  the  Old  World  from  Russia  to  China  and 
Japan,  and  likewise  in  North  America.  Another,  called  Sacam- 
mina,  abounds  as  aggregates  of  little  globular  bodies  in  some 
parts  of  the  limestone  of  Britain.  Corals  have  been  preserved 


Fig.   147. —  Carboniferous  Rugose  Corals,      (a)    Zaphrentis  Enniskilleni 
(i)  ;   (&)   Lithostrotion  junceum  (natural  size). 

in  prodigious  numbers;  indeed,  some  parts  of  the  limestone  are 
almost  entirely  made  up  of  them.  Most  of  them  are  rugose 
kinds,  characteristic  genera  being  Zaphrentis,  Lithostrotion, 
(Fig.  147),  ClisiopJiyllum,  Lonsdaleia.  With  these  there  occur 
also  tabulate  forms,,  including  Chcetetes,  Alveolites,  Favosites, 
etc.  Of  the  sea-urchins,  the  plates  and  spines  of  the  genus 
Archceocidaris  (Fig.  148)  are  specially  frequent.  But  the  most 
common  echinoderms  are  members  of  the  great  order  of  crinoids, 
which  must  have  grown  in  thick  groves  over  many  square  miles 
of  the  sea-bottom.  So  prodigiously  numerous  were  they  that 
their  remains  have  been  aggregated  into  beds  of  limestone 


288 


GEOLOGY 


hundreds  of  feet  in  thickness,  hence  known  as  crinoidal  or 
encrinite  limestone  (Fig.  76).  The  general  plant-like  form 
of  these  animals  is  shown  in  Fig.  149.  But  usually  the  cal- 
careous joints  and  plates  fell  asunder.  Frequent  genera  are 
named  Platycrinus,  Poteriocrinus,  Cyathocrinus.  The  Carbon- 


Fig.  148. —  Carbonifer- 
ous Sea-U  r  c  h  i  n 
(A  rchceocid  aris 
Vrei,  natural  size), 
(a)  Single  plate; 
(6)  Portion  of 
spine. 


Fig.  149. —  Carboniferous 
Crinoid  ( Woodocrinus 
expansus,  J). 


iferous  seas  were  tenanted  by  a  peculiar  extinct  order  of  echino- 
derms  known  as  Blastoids  or  Pentremites  (Fig.  150),  distin- 
guished from  true  crinoids  by  the  want  of  free  arms,  and  by 
the  arrangement  of  the  plates  forming  the  cup.  These  crea- 


Fig.  150. —  Carboniferous  Blastoid 
(Cup  of  Pentremite,  magnified), 
(a)  View  from  above;  (6)  Side 


Fig.  151.— Carbonifer- 
ous Trilobite  (Phil- 
lip s  i  a  der~biensis, 
natural  size). 


tures  are  characteristically  Carboniferous,  though  they  are 
found  also  in  the  higher  part  of  the  Silurian  system  and  in 
Devonian  rocks. 


THE  CARBONIFEROUS  PERIOD  289 

The  Crustacea  of  the  Carboniferous  period  presented  a  strong 
contrast  to  those  of  earlier  geological  time.  In  particular, 
the  great  family  of  the  Trilobites,  so  characteristic  of  the  older 
Palaeozoic  systems,  now  died  out  altogether.  Instead  of  its 
numerous  types  in  the  Silurian  and  Devonian  rocks,  it  is  repre- 
sented in  the  Carboniferous  system  by  only  four  genera,  all 
the  species  of  which  are  small  (Phillipsia,  Fig.  151,  Griffi- 
thides,  Brachymetopus) ,  and  none  of  which  rises  into  the  next 
succeeding  system.  The  most  abundant  crustaceans  were 
ostracods  —  an  order  still  abundantly  represented  at  the  pres- 
ent day.  They  are  minute  forms  enclosed  within  a  bivalve 
shell  or  carapace  which  entirely  invests  the  body.  Many  of 
these  live  in  fresh  water;  the  Cypris,  for  example,  being  abun- 
dant in  ponds  and  ditches.  Others 
are  marine,  while  some  are  brackish- 
water  forms.  In  the  Carboniferous 
lagoons,  as  at  the  present  time,  they 
lived  in  enormous  numbers ;  their  little 
seed-like  valves  are  crowded  together 
in  some  parts  of  the  shale  which  repre- 
sents the  mud  of  these  lagoons;  some- 
times they  even  form  beds  of  lime- 
stone. Doubtless,  they  served  as  food 
to  the  smaller  fishes  whose  remains  are 
usually  to  be  found  where  the  ostracod  Fi|;ol^n~ 
valves  are  plentiful.  One  of  the  prin-  Mcrrisii,  natural  size), 
cipal  genera  is  Leperditia.  There  were 

likewise  long-tailed  shrimp-like  crustaceans  (Anihrapalcemon, 
Palceocrangon) ,  and  king-crabs  (Prestwichid)  ;  while  in  the  ear- 
lier part  of  the  period  Eurypterids  still  survived  in  the  waters. 

Some  of  the  most  delicately  beautiful  fossils  of  the  Carbon- 
iferous limestone  belong  to  the  Polyzoa.  These  animals,  of 
which  familiar  living  examples  are  the  common  sea-mats  of 
our  shares,  are  characterised  by  their  compound  calcareous  or 
horny  framework  studded  with  minute  cells,  each  of  which  is 
occupied  by  a  separate  individual,  though  the  whole  forms 
one  united  colony.  One  of  the  most  abundant  Carboniferous 
genera  is  Fenestella  (Fig.  152).  So  numerous  are  the  polyzoa 
in  some  bands  of  limestone  as  to  constitute  the  main  part  of 
the  stone.  Their  delicate  lace-like  fronds  are  best  seen  where 


290 


GEOLOGY 


the  rock  has  been  exposed  for  a  time  to  the  action  of  the 
weather;  they  then  stand  out  in  relief  and  often  retain  per- 
fectly their  rows  of  cells. 


Fig.   153. —  Carboniferous   Brachiopods.     (a)    Productus   semireticulatus 
(5)  ;   (&)  Streptorhynchus  crenistria  (J)  ;   (c)  Spirifera  striata  (J). 

The  Brachiopods,  so  preponderant  among  the  molluscs  of 
the  earlier  division  of  Palaeozoic  time,   now  decidedly  wane 


Fig.   154. — Carboniferous  Lamellibranchs.     (a)    Edmondia  sulcata    (i)  ; 
(&)   Anthracomya  Adamsii   (§)  ;    (c)   Aviculopecten  fallax   (§). 

before  the  great  advance  of  the  more  highly  organised  lamelli- 
branchs  and  gasteropods.  Some  of  the  most  characteristic 
genera  (Fig.  153)  are  Productus,  Spirifera,  Streptorhynchus, 


THE  CARBONIFEROU     PERIOD 


291 


Ehynclionella,  Athyris,  Chonetes,  Terebratula,  Lingula,  Dis- 
cina.  Some  of  the  species  appear  to  range  over  the  whole  world, 
for  they  have  been  met  with  across  Europe,  in  China,  Aus- 
tralia, and  North  America.  Among  these  cosmopolitan  forms 
are  Productus  semireticulatus,  Productus  longispinus,  Strepto- 


Fig.  155. —  Carboniferous  Gasteropoda,     (a)    Euomphalns  pentangulatus 
(I)  ;   (&)  Bellerophon  tenuifascia  (§). 

rliynchus  crenistria,  Spirifera  glabm,  Terebratula  Jiastata  (Fig. 
153). 

Some  of  the  more  common  Lamellib ranch  molluscs  (Fig. 
154)  belong  to  the  genera  Aviculopecten,  Leda,  Nucula,  Ed- 
mondia,  Modiola,  Anthracomya.  Among  the  Gasterpods  Euom- 
phalus,  Pleurotomaria,  Loxonema,  and  Bellerophon  (Fig.  155) 
are  not  infrequent.  A  pteropod  (Conularia,  Fig.  156)  may  be 
gathered  in  great  numbers  in  some  parts  of  the  Carboniferous 


Fig.  156.— Carbon- 
iferous Pteropod 
(Conularia  quad- 
risulcata,  §). 


Fig.  157. —  Carboniferous  CepK- 
alopods.  (a)  Orthoceras  gold- 
fussianum  (i)  ;  (&)  Ooni- 
atites  sphcericus  (natural 
size). 


Limestone.     The  Cephalopods  were  represented  by  numerous 
species  of  Orthoceras,  Nautilus,  and  Goniatites  (Fig.  157). 

Eemains  of  fishes  are  not  infrequent  in  the  Carboniferous 
Limestone.  But  they  present  a  striking  contrast  to  those  of 
the  black  shales  and  ironstones  of  the  Coal-measures.  They 


292 


GEOLOGY 


consist  for  the  most  part  of  teeth  or  of  spines  belonging  to 
large  predatory  sharks.  These  teeth  were  placed  as  a  kind  of 
pavement  and  roof  in  the  mouth,  and  were  used  as  effective 
instruments  for  crushing  the  hard  parts  of  the  animals,  on 
which  these  larger  creatures  preyed.  If,  as  is  probable,  the 
sharks  fed  upon  the  ganoid  fishes  of  the  time,  they  must  have 
required  a  powerful  apparatus  of  teeth  for  crushing  the  hard, 
bony  armour  in  which  these  fishes  were  encased.  Of  the  com- 
moner genera  of  sharks,  which  have  been  named  from  the  forms 
of  their  teeth  —  the  only  hard  parts  of  their  structure  that 
have  survived  —  the  following  may  be  mentioned:  Cochliodus, 


Fig.  158. —  Carboniferous  Fishes,  (a)  Tooth  of  Rhisodus  Hiblerti  (J)  ; 
(&)  Tooth  of  Orodus  ramosus  (J)  ;  (c)  Ichthyodorulite  or  Fin-spine 
of  Pleuracanthus  Icevissimus  (J). 

Orodus,  Psammodus,  Petalodus.  The  small  ganoids  that  so 
abound  in  the  black  shales,  ironstones,  and  coal-seams  which 
represent  the  deposits  of  the  sheltered  lagoons  of  the  coal- 
jungles,  are  hardly  to  be  found  in  the  thick  limestone,  whence 
we  may  infer  that  they  were  inhabitants  of  the  quiet  shore- 
waters,  and  did  not  venture  out  into  the  open  sea,  where  the 
sharks  found  their  congenial  element.  But  the  occasional  occur- 
rence of  the  teeth  and  spines  of  sharks  in  the  Coal-measure 
shales  and  coal-seams  shows  that  these  monsters  now  and  then 
made  their  way  into  the  inland  waters,  where  they  would  find 
abundant  food. 


THE  CARBONIFEROUS  PERIOD 


293 


The  Carboniferous  system  iri  Europe  presents  at  least  two 
well-marked  subdivisions.  In  the  lower  section  the  strata  are 
in  large  measure  marine,  for  they  include  the  Carboniferous 
Limestone;  in  the  upper  part  they  consist  mainly  of  sand- 
stones, shales,  fire-clays,  and  coal-seams,  constituting  what  are 
called  the  Coal-measures,  or  coal-bearing  division  of  the  system. 
The  subjoined  Table  shows  the  order  of  succession  of  the  rocks 
in  Britain: — 

Coal-Measures. —  At  the  top,  red  and  gray  sandstones, 
clays,  and  thin  limestone,  resting  upon  a  great 
thickness  of  white,  gray,  and  yellow  sandstones, . 
clays,  shales,  and  fire-clays,  with  numerous  work- 
Lagoon  type.  -{  able  coal-seams,  and  with  a  lower  subdivision  of 
coal-bearing  beds,  among  which  there  occur  marine 
fossils  (Orthoceras,  Posidonomya,  etc.)  Thickness 
in  South  Wales,  12,000  feet;  South  Lancashire,  8,- 
000  feet;  Central  Scotland,  3,000  feet. 

Millstone  Grit. —  Grits,  flagstones,  sandstones,  and 
shales,  with  thin  seams  of  coal  and  occasional  bands 
containing  marine  fossils.  Thickness  400-1000  feet, 
increasing  in  Lancashire  to  5500  feet. 

Carboniferous  Limestone. —  Consisting  typically  of 
massive  marine  limestones  and  shales,  but  passing 
laterally  into  sandstones  and  shales,  with  thin  coal- 
seams,  which  indicate  alternations  of  marine  and 
brackish  water  conditions.  Thickness  in  South 
Wales,  500  feet,  increasing  northwards  to  more 
than  4000  feet  in  Derbyshire,  and  to  upwards  of 
6000  feet  in  Lancashire,  but  diminishing  north- 
wards into  Scotland. 

The  base  of  the  Carboniferous  Limestone  series 
passes  down  conformably  into  the  Upper  Old  Red 
Sandstone. 

The  Carboniferous  system  occupies  a  number  of  detached 
areas  on  the  European  continent.  Its  largest  tract  extends  from 
the  north  of  France,  through  Belgium,  into  Westphalia.  The 
most  important  coal-fields  of  Europe  belonging  to  this  system 
are  those  of  Belgium,  Westphalia,  the  north  of  France,  Saar- 
briicken,  St.  Etienne  in  Central  France,  Bohemia,  and  the 
Donetz  in  Southern  Eussia,  In  North  America,  the  Coal-meas- 
ures of  the  eastern  United  States  reach  a  thickness  of  4000 
feet  in  Pennsylvania,  and  contain  many  valuable  seams  of  coal. 
They  increase  in  thickness  northwards,  reaching  a  maximum 


Marine  type, 
but  passing 
northwar  d  s 
into  that  of 
the  lagoons. 


294  GEOLOGY 

of  8000  feet  in  Nova  Scotia.  They  are  underlain  by  bands  of 
conglomeratic  strata,  answering  to  the  English  Millstone  grit, 
below  which  comes  a  group  of  beds  with  marine  fossils  (sub- 
carboniferous),  probably  representing  the  Carboniferous  Lime- 
stone of  Europe.  In  Australia  and  New  Zealand  also  thick  masses 
of  sedimentary  strata  contain  recognisable  Carboniferous  organic 
remains.  In  New  South  Wales  they  include  a  valuable  suc- 
cession of  coal-seams. 


THE  PERMIAN  ROCKS  295 


CHAPTEE  XXI. 

THE  PERMIAN"  ROCKS. 

THE  prolonged  subsidence  during  which  the  Coal-measures 
were  accumulated  was  at  last  brought  to  an  end  by  a 
series  of  great  terrestrial  disturbances,  whereby  the 
lagoons  and  coal-growing  swamps  were  in  great  measure  effaced 
from  the  geography  of  Europe.  So  abrupt  in  some  regions  is 
the  discordance  between  the  Coal-measures  and  the  next  series 
of  strata,  that  geologists  have  naturally  been  led  to  regard  this 
break  as  one  of  great  chronological  importance,  serving  as 
the  boundary  between  two  distinct  systems.  Nevertheless,  so 
far  as  the  evidence  of  fossils  goes,  there  is  no  such  interruption 
of  the  Geological  Record  as  might  be  supposed  from  this  strati- 
graphical  unconformability,  many  of  the  Carboniferous  types 
of  life  having  survived  the  terrestrial  disturbances.  Again, 
though  the  discordance  among  the  strata  is,  in  many  parts  of 
Europe,  particularly  in  England,  most  striking,  yet  it  is  by  no 
means  universal.  On  the  contrary,  some  localities  (Autun  in 
France,  and  the  Bohemian  coal-field,  for  example)  escaped  the 
upheaval  and  prolonged  denudation  which  elsewhere  have  pro- 
duced so  marked  a  hiatus  in  the  chronicle.  And  in  these  places 
a  gradual  passage  can  be  traced  from  the  strata  and  fossils  of 
the  Coal-measures  into  those  of  the  next  succeeding  division 
of  the  series,  no  sharp  line  being  there  discoverable,  nor  any 
evidence  to  warrant  the  separation  of  the  overlying  strata  as 
an  independent  system  distinct  from  the  Carboniferous. 
Hence,  by  many  geologists,  the  rocks  now  to  be  described  are 
regarded  as  the  upper  part  of  the  Carboniferous  system. 

To  these  overlying  rocks  the  name  of  PERMIAN  was  given, 
from  the  Russian  province  of  Perm,  where  they  are  well  de- 
veloped. They  consist  of  red  sandstones,  marls,  conglomerates, 
and  breccias,  with  limestones  and  dolomites.  In  Germany  they 


296  GEOLOGY 

are  often  called  Dyas,  because  they  are  there  easily  grouped  in 
two  great  divisions.  The  coarsest  strata  —  breccias  and  con- 
glomerates —  are  composed  of  rounded  and  angular  fragments 
of  granite,  diorite,  gneiss,  graywacke,  sandstone,  and  other 
crystalline  and  older  Palaeozoic  rocks,  which  must  have  been 
upheaved  and  exposed  to  denudation  before  Permian  time.  The 
sandstones  are  usually  bright  brick-red  in  colour,  owing  to  the 
presence  of  earthy  peroxide  of  iron  which  serves  to  cement 
the  particles  of  sand  together.  The  shales  or  marls  are  coloured 
by  the  same  pigment.  So  characteristic  indeed  is  the  red 
colour  of  the  rocks  that  they  form  part  of  a  great  series  of 
strata,  originally  known  as  the  New  Red  Sandstone.  Generally, 
greenish  or  whitish  spots  and  streaks  occur  in  the  red  beds, 
marking  where  the  iron-oxide  has  been  reduced  and  removed 
by  decaying  organic  matter.  Eed  strata  are,  as  a  rule,  singularly 
barren  of  organic  remains,  probably  because  the  water  from 
which  the  iron-peroxide  was  precipitated  must  have  been  un- 
fitted for  the  support  of  life.  The  red  Permian  rocks  are  there- 
fore generally  unfossiliferous.  Among  them,  however,  occur 
dark  shales  or  "marl-slate,"  which  have  yielded  numerous  re- 
mains of  fishes.  The  limestones  too  are  f ossiliferous,  but  they 
are  associated  with  unfossiliferous  dolomite,  gypsum,  anhydrite, 
and  rock-salt.  In  some  places  seams  of  coal  also  occur. 

These  various  rocks  tell  distinctly  the  story  of  their  origin. 
They  could  not  have  been  deposited  in  the  open  sea,  but  rather  in 
basins  more  or  less  shut  off  from  it,  wherein  the  water  was 
charged  with  iron  and  was  liable  to  concentration,  with  the  con- 
sequent precipitation  of  its  solutions.  The  beds  of  anhydrite, 
gypsum,  and  rock-salt  are  memorials  of  these  processes.  The 
dolomite  may  at  first  have  been  laid  down  as  limestone  which 
afterwards  was  converted  into  dolomite  by  the  action  of  the 
magnesian  salts  in  the  concentrated  water.  In  such  intensely 
saline  and  bitter  solutions,  animal  life  would  not  be  likely  to 
flourish,  and  hence,  no  doubt,  the  poverty  of  fossils  in  the  Per- 
mian series  of  rocks.  But  it  is  observable  that  where  evidence 
occurs  of  the  cessation  of  ferruginous,  saliferous,  and  gypseoup 
deposition,  fossils  not  infrequently  appear.  The  brown  Marl- 
Slate,  for  example,  and  the  thick  beds  of  limestone  are  some- 
times abundantly  f  ossiliferous,  and  indeed  are  almost  the  only 
bands  of  rock  in  the  whole  series  where  organic  remains  occur. 


THE  PERMIAN  ROCKS 


297 


They  were  probably  deposited  during  intervals  when  the  bar- 
riers of  the  inland  seas  or  salt-lakes  were  broken  down,  or,  at 
least,  when  from  some  cause  the  waters  came  to  be  connected 
with  the  open  sea,  and  when  a  portion  of  the  ordinary  marine 
fauna  swarmed  into  them.  Volcanic  action  showed  itself  during 
Permian  time  in  many  parts  of  Western,  Central,  and  Southern 
Europe.  There  was  a  group  of  small  volcanoes  in  the  south  of 
Scotland.  Great  eruptions  took  place  in  Germany,  notably  in 
the  area  of  the  present  Yosges  mountains,  and  the  region  of 
volcanic  activity  extended  across  the  region  where  now  the  Alps 


Fig.    159. —  Permian   Plants,      (a)    Callipteris  conferta    (4);    (&)    Wai- 
chia  piniformis   (i). 

stand,  as  far  south  as  Cannes  on  the  shores  of  the  Mediterranean. 
Hence,  from  the  very  circumstances  in  which  their  remains 
have  been  entombed  and  preserved,  the  flora  and  fauna  of  Per- 
mian times  are  comparatively  little  known.  The  total  number 
of  species  and  genera  obtained  from  Permian  rocks,  hardly  more 
than  300  in  all,  forms  a  singular  contrast  to  the  ample  assem- 
blages which  have  been  recovered  from  the  older  systems.  But 
that  the  land  of  these  times  was  still  richly  clothed  with  vegeta- 
tion and  the  sea  abundantly  stocked  with  animal  life,  there  can 
be  no  doubt.  The  flora  appears  to  have  closely  resembled  that 


298 


GEOLOGY 


of  the  Carboniferous  period,  a  considerable  proportion  of  the 
species  of  plants  being  survivals  from  the  Carboniferous  jungles 
and  forests.  The  Lepidodendra,  Sigillariae,  and  Calamites, 
tvhich  had  been  such  conspicuous  members  of  all  the  Palaeozoic 
floras,  now  appear  in  diminishing  number  and  variety,  and  final- 
ly die  out.  With  their  cessation,  new  features  arise  in  the 
vegetation.  Among  these  may  be  mentioned  the  abundance  of 
tree-ferns,  which,  though  they  sparingly  existed  even  as  far  back 
as  Devonian  times,  now  attained  a  conspicuous  development 
(Psaronius,  Caulopteris) .  The  genus  of  ferns  called  Callip- 
teris  likewise  played  a,  prominent  part  in  the  Permian  wood- 


Fig.   160. —  Permian   Brachiopods.      (a)    Producing  horridus    (reduced); 
(6)  Strophalosia  Ooldfussi;  (c)  Camarophoria  humbletonensis  (§). 

lands  (Fig.  159,  a).  But  perhaps  the  most  remarkable  feature 
in  the  flora  was  the  abundance  of  its  conifers,  and  the  appearance 
of  the  earliest  forms  of  cycads.  The  yew-like  conifer  Walchia 
(Fig.  159,  &)  if  we  may  judge  from  the  abundance  of  its  re- 
mains, flourished  in  great  profusion  on  the  drier  grounds, 
mingled  with  others  that  bore  cones  (Utttnannia) .  The  cycads, 
which  now  made  their  advent,  continued  during  Mesozoic  time 
to  give  the  leading  character  to  the  vegetation  of  the  globe. 

The  scanty  relics  of  the  Permian  fauna,  as  above  stated,  have 
been  almost  wholly  preserved  in  those  strata  which  were  de- 
posited during  temporary  irruptions  of  the  open  sea  into  the 


THE  PERMIAN  ROCKS  299 

inland  salt-basins  of  the  time.  Some  of  the  Carboniferous 
genera  of  brachiopods  still  survived  —  Productus,  Spirifera,  and 
Strophalosia  being  conspicuous  (Fig.  160).  Among  the  lamel- 
libranchs  Axinus,  Bakevellm,  and  Schizodus  are  frequent  forms 


Fig.   161. —  Permian   Lamellibranchs.     (a)    Bakevellia,  tumida    (natural 
.size)  ;    (&)   Schizodus  Schlotheimi  (natural  size). 

(Fig.  161).  Among  the  higher  molluscs,,  which  have  been  but 
sparingly  preserved  in  the  rocks,  the  old  types  of  OrtJioceras, 
Cyrtoceras,  and  Nautilus  are  still  to  be  noticed.  In  Europe,  the 
fishes  of  the  time  have  been  chiefly  sealed  up  in  the  marl-slate 
or  copper-shale  (Kupferschiefer) ;  two  of  the  most  frequent 
genera  being  Palceoniscus  and  Platysomus  (Fig.  162). 


Fig.  162. —  Permian  Ganoid  Fish  (Platysomus  striatus,  i). 

Labyrinthodonts  continued  to  abound  in  the  waters.  Some 
of  the  Carboniferous  genera  still  survived,  but  with  these  were 
associated  many  new  forms,  most  of  which  have  been  discovered 
in  the  strata  overlying  the  true  Coal-measures  of  Bohemia  (Fig. 
163).  But  a  great  onward  step  in  the  advance  of  animal  organ- 
isation was  made  in  Permian  time  by  the  appearance  of  the 


300  GEOLOGY 

earliest  known  lizard  —  Proterosaurus,  which,  like  the  living 
crocodile,  had  its  teeth  implanted  in  distinct  sockets. 

In  Britain  the  Permian  strata  rest  unconformably  on  the 
Carboniferous  system,  which  must  have  been  greatly  disturbed 


Fig.    163. —  Permian    Labyrinthodont    (Branchiosaurus    salamandroides, 

natural  size). 

and   enormously  denuded  before  they  were  deposited.     They 
consist  of  the  following   subdivisions: — 

Upper  red  sandstones,  clays  and  gypsum  (50  to  100  feet  thick  in  the 
east  of  England,  but  swelling  out  west  of  the  Pennine  Chain  to 
600  feet). 

Magnesian  limestone  —  a  mass  of  dolomite  ranging  up  to  600  feet  in 
thickness,  and  the  chief  repository  of  the  Permian  fossils;  remark- 
able for  the  curious  concretionary  forms  assumed  by  many  of  its 
beds  on  the  coast  of  Durham  (Fig.  75).  [Zechstein  of  Germany.] 

Marl-slate  —  a  hard  brown  shale  with  occasional  limestone  bands.  [Kup- 
f erschiefer] . 

Lower  red  and  variegated  sandstones  with  conglomerates  and  breccias. 
This  division  attains  a  thickness  of  3000  feet  in  Cumberland,  but  is 
hardly  represented  in  the  east  of  England.  [Rothliegende  of  Ger- 
many.] 

In  Germany,  where  the  Dyas  or  twofold  development  of  the 
Permian  rocks  is  so  well  displayed,  the  lower  subdivision,  called 
Eothliegende,  consists  of  great  masses  of  conglomerate  with 
sandstones,  shales,  thin  limestones,  and  important  intercalations 
of  contemporaneous  volcanic  rocks,  both  lavas  and  tuffs.  The 
upper  section  is  composed  chiefly  of  limestone  called  Zechstein, 
and  answering  to  the  Magnesian  limestone  of  England.  With 
it  are  associated  beds  of  anhydrite,  gypsum,  rock-salt,  and 
bituminous  limestone,  and  underneath  it  lies  the  celebrated 
Kupferschiefer  or  copper-shale  —  a  black  bituminous  shale, 
about  two  feet  thick,  which  has  long  been  extensively  worked 
on  the  flanks  of  the  Harz  Mountains  for  the  ores  of  copper  with 


THE  PERMIAN  ROCKS  301 

which  it  is  impregnated,  and  which  is  the  great  repository  for 
the  fossil  fishes  of  the  Permian  period.  This  remarkable  band 
of  rock  was  probably  deposited  in  one  of  the  inland  basins,  which 
at  first  may  have  maintained  a  free  communication  with  the 
open  sea.  But  eventually  mineral  springs,  not  improbably  con- 
nected with  the  volcanic  action  of  the  time,  brought  up  such  an 
abundant  supply  of  dissolved  metallic  salts  as  to  kill  the  fish 
and  render  the  water  unsuitable  for  their  existence.  The 
metallic  salts  were  reduced  and  precipitated  as  sulphides  round 
the  organisms,  and  impregnated  the  surrounding  mud.  In  the 
overlying  succession  of  strata,  we  see  how  the  area  was  once 
more  overspread  by  the  clearer  and  opener  water,  which  brought 
in  the  fauna  of  the  Zechstein,  and  then  how  the  basin  gradually 
came  to  be  shut  off  once  more,  until  from  its  concentrated 
waters  the  various  beds  of  anhydrite,  gypsum,  and  rock-salt  were 
thrown  down. 

In  the  heart  of  France,  at  Autun,  the  Coal-measures  pass  up- 
ward into  Permian  strata,  as  already  stated.  That  area  appears 
to  have  escaped  the  disturbance  which  in  Western  Europe  placed 
the  Permian  unconformably  upon  the  Carboniferous  rocks.  It 
presents  a  mass  of  sandstones,  shales,  coal-seams,  and  some  bands 
of  magnesian  limestone,  the  whole  having  a  thickness  of  more 
than  3,000  feet  referred  to  the  Permian  system.  The  plants  in 
the  lower  part  of  this  group  of  strata  are  unmistakably  Car- 
boniferous, but  Permian  forms  appear  in  increasing  numbers  as 
the  beds  are  followed  upwards  until  the  highest  stage  presents  a 
predominant  Permian  flora.  Besides  the  characteristic  Permian 
fishes  these  strata  have  yielded  remains  of  several  salamander- 
like  animals  (Protriton  or  Branchiosaurus,  Melanerpeton),  and 
of  some  labyrinthodonts  (Actinodon,  Euchirosaurus,  etc.). 


302  GEOLOGY 


CHAPTEK  XXII. 

MESOZOIC    PERIODS  —  TRIASSIO. 

THE  great  series  of  red  strata  referred  to  in  the  foregoing 
chapter  as  overlying  the  Carboniferous  system  in  England 
was  called  "  New  Eed  Sandstone,"  to  distinguish  it  from 
the  "  Old  Eed  Sandstone "  which  underlies  that  system.  But 
the  progress  of  geology  on  the  European  continent  eventually 
proved  that,  notwithstanding  their  general  similarity  of  litho- 
logical  character,  two  series  of  rocks  had  been  comprised  under 
the  general  title  of  New  Eed  Sandstone.  The  older  of  these, 
separated  from  the  rest  under  the  name  of  Permian,  was  placed 
at  the  top  of  the  great  succession  of  Paleozoic  formations.  The 
younger  division  (still  sometimes  spoken  of  as  New  Eed  Sand- 
stone) was  called  Trias,  and  was  regarded  as  the  first  system 
in  the  great  Mesozoic  or  Secondary  succession. 

Essentially  the  Permian  strata  form  merely  the  upper  part  of 
the  Carboniferous  system.  Their  types  of  life  are  fundamentally 
Palaeozoic,  but,  as  we  have  seen,  both  the  flora  and  fauna  are 
marked  by  a  decrease  in  the  number  and  variety  of  old  forms, 
and  by  the  advent  of  the  precursors  of  a  new  order  of  things. 
Conifers  and  cycads  now  began  to  replace  the  early  types  of 
lepidodendron  and  sigillaria;  amphibians  became  more  abun- 
dant, and  saurians  now  took  their  place  at  the  head  of  the 
animal  world. 

But  when  we  ascend  into  the  Trias,  though  in  Europe  the 
physical  conditions  of  deposition  remained  much  the  same  as 
in  Permian  time,  we  meet  with  a  decided  contrast  in  the 
organic  remains.  A  new  and  more  advanced  phase  of  develop- 
ment presents  itself  in  that  richer  and  more  varied  assemblage 
of  plant  and  animal  life  which  characterised  Mesozoic  or  Sec- 
ondary time. 

The  word  TRIAS  has  reference  to  the  marked  threefold  division 
of  the  rocks  of  this  system  in  Germany.  In  that  country,  and 


MESOZOIC  PERIODS  — TRIASSIC  303 

generally  in  Western  Europe,  the  rocks  consist  of  bright  red 
sandstones  and  marls  or  clays,  with  beds  of  gypsum,  anhydrite, 
rock-salt,  dolomite,  and  limestone.  These  rocks,  so  closely  re- 
sembling the  Permian  series  below,  had  evidently  a  similar 
origin.  They  were  in  large  part  deposited  in  inland  seas  or 
salt-lakes,  wherein,  by  evaporation  and  concentration  of  the 
water,  the  dissolved  salts  were  precipitated  upon  the  bottom,  and 
where,  consequently,  the  conditions  must  have  been  extremely 
unfavourable  for  the  presence  of  living  things.  The  sites  of 
these  inland  basins  can  still  be  partially  traced.  They  extended 
at  least  as  far  west  as  the  north  of  Ireland.  One  or  more  of 
them  lay  across  the  site  of  the  plains  of  Central  England. 
Others  were  dotted  over  the  lowlands  of  middle  Europe.  The 
largest  of  them  occupied  an  extensive  area  now  traversed  by 
the  Ehine.  It  stretched,  on  the  one  hand,  from  Basel  to  the 
plains  of  Hanover,  and,  on  the  other,  from  the  highlands  of 
Saxony  and  Bohemia  across  the  site  of  the  Vosges  Mountains 
westward  to  the  flank  of  the  Ardennes.  The  continent  must 
then  have  been  somewhat  like  the  steppes  of  Southern  Eussia  — 
.  a  region  of  sandy  wastes  and  salt-lakes,  with  a  warm  and  dry 
climate.  Probably  higher  land  rose  to  the  north,  as  in  earlier 
geological  times,  for  traces  of  its  vegetation  have  been  found  in 
Sweden.  But  southwards  lay  the  more  open  sea,  spreading 
over  part  at  least  of  the  site  of  the  modern  Alps,  and  thence 
probably  across  much  of  Asia  to  the  Indian  and  Pacific  Oceans. 
So  long  as  only  the  deposits  of  the  salt-basins  had  been  ex- 
plored, it  was  but  natural  that  comparatively  little  should  be 
known  of  the  flora  and  fauna  of  the  Triassic  period.  The 
climate  around  these  lakes  was  perhaps  not  a  very  salubrious 
one,  and  hence  there  may  have  been  only  a  scanty  terrestrial 
fauna  in  their  immediate  vicinity,  while  the  waters  of  the  lakes 
themselves  were  unsuited  for  the  support  of  life.  It  is  not 
surprising,  therefore,  that  the  strata  deposited  in  these  tracts  are 
on  the  whole  unf ossilif erous ;  that,  indeed,  fossils  only  abound 
where  there  are  indications  that,  owing  to  some  temporary  de- 
pression or  breaking  down  of  the  barriers,  the  open  sea  spread 
into  these  basins,  and  carried  with  it  the  organisms  whose  re- 
mains gathered  into  beds  of  limestone.  But  over  the  tracts 
that  lay  under  the  open  sea,  a  more  abundant  marine  fauna 
lived  and  died.  It  is  in  the  records  of  that  sea-bottom,  rather 


304 


GEOLOGY 


than  in  those  of  the  salt-basins,  that  we  must  seek  for  the  evi- 
dence of  the  general  character  of  the  life  over  the  globe,  and 
for  the  fossil  data  with  which  to  compare  together  the  Triassic 
rocks  of  distant  regions. 

There  are  traces  of  contemporaneous  volcanic  action  among 
the  Triassic  strata.  A  little  group  of  volcanoes  appears  to  have 
existed  during  Triassic  time  in  South  Devonshire;  but  in  the 
region  of  the  Eastern  Alps,  especially  around  Predazzo  *in  the 
Tyrol,  evidence  of  far  more  extensive  eruptions  exists. 

The  flora  of  the  Triassic  period  has  been  preserved  chiefly  in 
the  dark  shales  and  thin  coal-seams  formed  in  some  of  the  inland 


Fig.  164. —  Triassic  Plants,  (a)  Horse-tail  Reed  (Equisetum  columnare, 
I)  ;  (6)  Conifer  (Voltzia  heterophylla,  £)  ;  (c)  Cycad  (Pterophyl- 
lum  Blasii,  |). 

basins.  So  far  as  known  to  us  it  consisted  chiefly  of  ferns, 
equisetums  or  horse-tails,  conifers,  and  cycads.  Among  the 
ferns  a  few  Carboniferous  genera  still  survived,  but  some  of  the 
most  characteristic  forms  were  tree-ferns.  The  oldest  known 
true  horse-tails  are  met  with  in  the  Trias  (Fig.  164,  a).  The 
most  abundant  conifer  is  the  cypress-like  Vollzia  (Fig.  164,  I). 
Cycads,  already  a  feature  in  the  vegetation  of  the  Permian  sys- 
tem, now  increase  in  number  and  variety.  During  the  Mesozoic 
ages  they  continued  to  be  the  most  characteristic  members  of  the 
terrestrial  flora,  insomuch  that  this  division  of  geological  time 
is  sometimes  spoken  of  as  the  "Age  of  Cycads/'  Some  of  the 


MESOZOIC  PERIODS  —  TRIASSIO  305 

more  common  cycads  in  the  Triassic  rocks  are  Pterophyllum, 
Zamites,  and  Podozamites  (Fig.  164,  c). 

The  red  gypseous  and  saliferous  strata,  for  the  reason  already 
given,  are  on  the  whole  unfossiliferous.  Here  and  there,  foot- 
prints of  amphibians,  preserved  on  the  sandstones,  give  us  a 
glimpse  of  the  higher  forms  of  life  that  moved  about  on  the 
margin  of  the  salt-lakes.  The  beds  of  limestone,  which  repre- 
sent intervals  when,  for  a  time,  the  sea  over- 
spread the  lakes,  contain  sometimes  abundant 
fossils.  But  they  are  numerous  in  individuals 
rather  than  in  species  or  genera,  as  if  the  con- 
ditions for  life  in  those  waters  were  still  some- 
what unfavourable.  On  the  other  hand,  the 
limestones  laid  down  in  the  opener  sea  are 
crowded  with  a  varied  fauna.  One  of  the  most 
typical  fossils  of  the  Trias  is  the  crinoid  En- 
crinus  liliiformis  (Fig.  165),  one  of  the  most 
familiar  fossils  of  the  limestones  (Muschelkalk) 
which  in  Germany  form  the  central  division  of 
the  system.  Among  the  lamellibranchs,  Myo- 
phoria,  Avicula,  Pecten,  Cardium,  Pullastra, 
Daonella,  and  Monotis  are  characteristic  genera  (Fig.  166), 
some  species  such  as  Avicula  contorta,  Pecten  valoniensis, 
and  Cardium  rhceticum  being  eminently  useful  in  tracing 
the  upper  parts  of  the  Trias  (Kha3tic)  all  over  Europe 
from  Italy  to  Scandinavia.  One  of  the  most  distinctive  features 
of  the  Triassic  fauna  is  its  development  of  cephalopod  life.  In 
the  limestones  of  the  middle  subdivision  in  Germany,  a  few 
species,  of  cephalopods  occur,  the  two  prevalent  forms  being 
species  of  Nautilus  and  the  ammonite  Ceratites  (Fig.  167). 
But  when  we  turn  to  the  Trias  of  the  Eastern  Alps,  which  repre- 
sents the  deposits  of  the  more  open  sea,  we  meet  with  a  remark- 
able abundance  and  variety  of  cephalopods,  and  with  a  striking 
admixture  of  ancient  and  more  modern  types.  For  example, 
the  venerable  genus  Orthoceras,  which  occurs  even  down  in  the 
Cambrian  rocks,  is  found  also  here  as  a  survival  from  Pabeozoic 
time.  But  new  types  now  appeared.  In  particular,  the  tribe 
of  Ammonites,  so  pre-eminently  typical  of  the  molluscan  life 
of  the  Mesozoic  •  seas,  is  represented  by  numerous  genera  and 
species  (Arcestes,  Tracliyceras,  Pinacoceras,  Pliylloceras,  be- 


306 


GEOLOGY 


sides  Ceratites  above  referred  to).  Among  the  fishes  of  the 
Trias,  the  genera  Acrodus,  Ceratodus,  Gyrolepis,  Hy~bodus,  and 
Pholidophorus  may  be  mentioned.  Labyrinthodonts  still 
haunted  the  lagoons  and  sandy  shores  (Mastodonsaurus,  Trema- 
tosaurus) ;  but  they  no  longer  remained  the  most  important 


Fig.  166. —  Triassic  Lamellibranchs.  (c)  Avicula  contorta  (natural 
size)  ;  (6)  Pecten  valoniensis  (i)  ;  (c)  Cardium  rhceticum  (natural 
size)  ;  (d)  Myophoria  vulgaris  (i). 

members  of  the  animal  world.  Various  early  types  of  lizards 
now  took  their  places  in  the  ranks  of  creation  (Hyperodapedon, 
Telerpeton,  Fig.  168).  A  strange  order  of  Triassic  reptiles  was 


Fig.    167. —  Triassic   Cephalopoda,     (a)    Nautilus   bidorsatus    Ck)  ',    (&) 
Ceratites  nodosus  (reduced). 

characterised  by  the  jaws  having  the  form  of  a  beak,  somewhat 
like  that  of  a  turtle ;  Dicynodon,  one  of  these  forms,  carried  two 
huge  tusks  in  the  upper  jaw.  A  remarkable  and  long-extinct 


MESOZOIC  PERIODS  — TRIASSIC 


307 


order  of  reptiles,  that  of  the  Deinosaurs,  made  its  first  appear- 
ance in  Triassic  time.  These  creatures  were  marked  by  peculi- 
arities of  structure  that  linked  them  both  with  true  reptiles 
and  with  birds,  while  in  size  they  sometimes  resembled  ele- 


Fig.  168.—  Trias- 
sic Lizard  (Te- 
lerpeton  elginen- 
se,  1). 


Fig.  169. —  Triassic  Croco- 
dile (Scutes  of  Stagonole- 
pis  elginensis,  J). 


phants  and  rhinoceroses.  They  seem  to  have  walked  mainly 
on  their  hind  feet,  the  three-toed  or  five-toed  bird-like  imprints 
of  which  are  numerous  on  some  beds  of  sandstone.  They  are 
characteristically  Mesozoic  types  of  life.  An- 
other not  less  typically  Mesozoic  form,  that 
of  the  Plesiosaurs,  likewise  began  in  Triassic 
time ;  but  it  will  be  more  particularly  alluded 
to  in  the  following  chapter.  The  earliest 
known  crocodiles  have  been  found  in  Triassic 
rocks;  some  of  the  scutes  or  scales  of  one 
of  these  animals  are  shown  in  Fig.  169.  But 
the  most  important  advance  in  the  fauna  of 
the  globe  during  the  Triassic  period  was  the 
first  appearance  of  mammalian  life.  De- 
tached teeth  and  lower  jaws  have  been  met 
with  in  the  uppermost  parts  of  the  Triassic 
system,  which  have  been  identified  as  possess- 
ing structures  like  those  of  some  of  the  (  T  )  • 
marsupial  animals  of  Australia  (Microlestcs,  Fig.  170).  It  is 
interesting  to  know  that  the  earliest  representatives  of  the  great 
class  of  the  mammalia  belonged  to  one  of  its  lowest  divisions. 
They  were  small  creatures  probably  resembling  the  OrnitJio- 
rliynclius  and  Echidna  of  Australia. 


Fig.  170. —  Trias- 
sic Marsupial 
(M  i  c  r  o  I  e  s  - 
tes  Moorei). 
(o)  Lower  mo- 
lar tooth,  outer 
side  (|  )  ;  (6) 
Ditto  (nat. 
size)  ;  (c)  Dit- 
to, front  side 


308 


GEOLOGY 


The  Triassic  strata  of  the  inland  basins  (England,  Germany, 
France,  etc.)  have  been  subdivided  into  the  following  groups: — 

Red,  green,  and  gray  marls,  black  shales,  sandstones, 
bone-beds,  and  in  Germany  sometimes  thin  seams 
of  coal.  Characteristic  fossils  are  Gardium  rhceti- 
cum,  Avicula  contorta,  Pecten  valoniensis,  Pullastra 
arenicola,  Acrodus,  Ceratodus,  Hybodus,  Saurians, 
Microlestes. 

Red,  gray,  and  green  marls,  with  beds  of  rock-salt  and 
gypsum. 

Red  sandstones  and  marls  (England)  ;  gray  sand- 
stones and  dark  marls  and  clays,  with  thin  seams 
of  earthy  coal  (Germany). 

Limestones  and  dolomites,  with  bands  of  anhydrite, 
gypsum,  and  rock-salt.  The  limestones  are  the 
great  repository  of  the  fossils.  This  subdivision  is 
absent  or  only  feebly  represented  in  England. 

Mottled  red  and  green  sandstones,  marls,  and  some- 
times pebble-beds. 


Rhsetic. 


Keuper    or    up- 
per    Trias. 


Muschelkalk    or 
Middle   Trias. 

Bunter  or  Low- 
er Trias. 


The  salt-beds  of  Cheshire  have  long  been  worked  for  com- 
mercial purposes.  The  lower  bed  is  sometimes  more  than  100 
feet  thick;  but  the  salt  deposits  of  Germany  are  much  more 
important.  Thus  at  Sperenberg,  20  miles  south  of  Berlin,  a 
boring  was  put  down  through  about  290  feet  of  gypsum,  and 
then  through  upwards  of  5,000  feet  of  rock-salt,  without  reach- 
ing the  bottom  of  the  deposit. 

The  alternation  of  bands  of  rock-salt  with  thin  layers  of 
anhydrite  or  of  gypsum  no  doubt  marks  successive  periods  of 
desiccation  and  inflow;  in  other  words,  each  seam  of  sulphate 
of  lime  (which  is  the  least  soluble  salt,  and  is  therefore  thrown 
down  first)  seems  to  indicate  a  renewed  supply  of  salt  water  from 
outside,  probably  from  the  open  sea,  while  the  overlying  rock- 
salt  shows  continued  evaporation,  during  which  the  water  be- 
came a  concentrated  solution  and  deposited  a  thicker  layer  of 
sodium  chloride.  Sometimes  the  concentration  continued  until 
still  more  soluble  salts,  such  as  chlorides  of  potassium  and  mag- 
nesium, were  also  eliminated.  These  phenomena  are  well  dis- 
played at  the  great  salt-mines  of  Stassfurt,  on  the  north  flank 
of  the  Harz  Mountains.  The  lowest  rock  there  found  is  a  mass 
of  pure,  solid,  crystalline  rock-salt  of  still  unknown  thickness, 
but  which  has  been  pierced  for  about  1,000  feet.  This  rock 
is  separated  into  layers,  averaging  about  3%  inches  in  thick- 


MESOZOIC  PERIODS  — TRIASSIC  309 

ness,  by  partings  of  anhydrite  %  inch  thick  or  less.  If  each 
of  these  "year  rings,"  as  the  German  miners  call  them,  repre- 
sented the  deposit  formed  during  the  dry  season  of  a  single 
year,  then  the  mass  of  1,000  feet  would  have  taken  more  than 
3,000  years  for  its  formation.  But  there  do  not  appear  to  be 
any  good  grounds  for  believing  that  each  band  marks  one  year's 
accumulation.  Above  the  rock-salt  lie  valuable  deposits  of  the 
more  soluble  salts,  particularly  chlorides  of  potassium  and  mag- 
nesium, with  sulphates  of  lime  and  magnesia.  The  compound 
known  as  Carnallite  (a  double  chloride  of  potassium  and  mag- 
nesium) is  now  the  chief  source  of  the  potash  salts  of  commerce. 

In  the  Ehaetic  beds  of  England,  one  of  the  most  interesting 
bands  is  the  so-called  "bone-bed" — a  thin  layer  of  dark  sand- 
stone, charged  with  bones,  teeth,  and  scales  of  fishes  and  sauri- 
ans.  A  thin  seam  of  limestone  in  the  same  group  contains 
wings  and  wing-cases  of  insects. 

The  Trias  of  the  Eastern  Alps  reaches  a  thickness  of  many 
thousand  feet,  and  forms  great  ranges  of  mountains.  The  lower 
division  runs  throughout  the  Alps  with  considerable  uniformity 
of  character,  so  that  it  forms  a  useful  platform  from  which  to 
investigate  the  complicated  geological  structure  of  these  moun- 
tains. The  Upper  Trias  consists  of  several  thousand  feet  of 
shales,  marls,  limestones,  and  dolomites,  while  the  Ehaetic  group 
swells  out  into-  a  great  succession  of  limestones  and  dolomites. 
During  the  time  when  the  Triassic  sea  stretched  over  the  site 
of  the  Alps  there  were  evidently  considerable  oscillations  of 
level,  and  there  likewise  occurred  extensive  volcanic  eruptions, 
whereby  large  masses  of  lavas  and  tuffs  were  ejected.  These 
rocks  now  form  conspicuous  hills  in  the  Tyrol. 

Triassic  rocks  have  been  traced  in  Beloochistan,  the  salt  range 
of  the  Punjab,  Northern  Kashmir,  and  Western  Thibet.  They 
cover  a  large  area  of  North  America,  and  have  been  recognised 
in  Australia  and  New  Zealand.  Eocks  which  have  been  assigned 
to  the  same  geological  period  occur  in  South  Africa,  and  have 
there  yielded  a  remarkable  series  of  reptilian  remains. 


310  GEOLOGY 


CHAPTEE  XXIII. 

THE  JURASSIC  PERIOD. 

THE  system  which  follows  the  Trias  has  received  its  name, 
JURASSIC,  from  the  Jura  Mountains,  where  it  is  well 
developed.  It  contains  the  record  of  a  great  series  of 
geographical  changes,  which  in  Europe  entirely  effaced  the 
inland  basins  and  sandy  wastes  of  the  previous  period,  and  dur- 
ing which  sedimentary  rocks  were  accumulated  that  now  extend 
in  a  broad  belt  across  England,  from  the  coasts  of  Dorset  to 
those  of  Yorkshire,  cover  an  enormous  area  of  France  and  Ger- 
many, and  sweep  along  both  sides  of  the  Alps  and  the  Apennines. 
These  strata  vary  greatly  in  composition  and  thickness  as  they 
are  traced  from  country  to  country.  In  one  district  they  pre- 
sent a  series  of  limestones  which,  as  they  are  followed  into  an- 
other area,  pass  into  shales  or  sandstones.  The  widespread  uni- 
formity of  lithological  character,  so  marked  among  the  Palaeozoic 
systems,  gives  place  in  the  Mesozoic  series  to  greater  variety. 
Sandstones,  shales,  and  limestones  alternate  more  rapidly  with 
each  other,  and  are  more  local  in  their  extent.  They  indicate 
greater  vicissitudes  in  the  process  of  deposition,  more  frequent 
alternations  of  sea  and  land,  and  not  improbably  greater  differ- 
ences of  climate  than  in  Palaeozoic  time. 

The  flora  of  the  Jurassic  period  is  marked  by  the  same  general 
characters  as  that  of  the  Trias  —  ferns,  equisetums,  conifers,  and 
cycads,  being  its  distinguishing  elements.  Cycads  now  abound 
(Pterophyllum,  Zamites,  Cycadites,  and  many  others,  Fig.  171). 
Among  the  conifers  are  the  remote  ancestors  of  our  "  Puzzle- 
monkeys,"  introduced  from  Chili  and  now  so  common  as  orna- 
mental garden  shrubs  (Araucaria  imbricata),  and  of  our  pines 
and  firs.  This  vegetation  flourished  luxuriantly  over  the  area 
of  Britain;  on  the  site  of  Yorkshire  it  grew  so  densely  as  to 
give  rise  to  thick  peaty  accumulations,  which  now  form  beds 


THE  JURASSIC  PERIOD 


311 


of  coal.  It  went  far  northward,  for  its  remains  have  been  abun- 
dantly preserved  in  Spitzbergen,  where  numerous  cycads  have 
been  found  among  them.  These  plants  unquestionably  grew 
and  flourished  within  the  Arctic  Circle,  so  that,  though  the 
climates  of  the  globe  were  already  beginning  to  emerge  from 


Fig.  171. —  Jurassic  Cycad  (Cycadeoidea,  microphylla,  J) 

the  greater  uniformity  of  Palaeozoic  time,  the  Arctic  regions  still 
enjoyed  a  temperature  like  that  of  sub-tropical  countries  at  the 
present  time. 

The  animal  world  during  the  Jurassic  period,  if  we  may  judge 
of  it  from  its  fossil  remains,  must  have  been  much  more  varied 
alike  on  land  and  in  the  sea  than  during  the  previous  ages  of  the 
earth's  history.  From  the  circum- 
stances in  which  the  strata  were  de- 
posited, relics  of  the  life  of  the  land 
are  frequently  met  with,  besides 
abundant  records  of  that  of  the  sea. 
A  characteristic  feature  of  the  period 
was  the  profusion  of  corals,  which 
at  different  times  spread  over  much 
of  the  site  of  modern  Europe.  They 
were  no  longer  the  rugose  forms 
so  distinctive  of  the  Paleozoic  seas, 
but  true  reef-building  astraeids,  be- 
longing to  the  genera  Isastrcea,  Thamnastrcea,  Montlivaltia,  The- 
cosmilia,  etc.  (Fig.  172).  Crinoids  were  still  abundant,  though 
less  so  than  in  the  Carboniferous  limestone  sea ;  the  old  forms  were 


Fig.  172. —  Jurassic  reef- 
building  Coral  ( Isastrcea 
explanata,  J).  From  the 
Corallian. 


312 


GEOLOGY 


now  replaced  by  others,  among  which  the  most  conspicuous  was 
the  Pentacrinite  (Fig.  173) — a  genus  still  living  in  our  present 
seas.  Sea-urchins  swarmed  on  some  parts  of  the  sea-floor; 


Fig.  173. —  Jurassic  Crinoid  (Pentacrinus  fasciculosus,  §). 

among  their  more  frequent  genera  are  Cidaris  (Fig.  174),  Dia- 
dema,  Hemicidaris,  Acrosalenia,  Glyptichus,  Pygaster.     Of  the 

contrasts  between  the  Mesozoic  and 
Paleozoic  faunas,  one  of  the  most 
marked  is  to  be  found  among  the 
brachiopods.  Except  the  persistent  in- 
articulate types  which  have  lived  on 
from  Cambrian  time  to  the  present  day 
(Crania,  Lingula,  Discina),  the  num- 
reef_  erous  and  varied  forms  which  played  so 
important  a  part  in  the  life  of  the 
Palaeozoic  seas  died  out  almost  entirely 
at  the  close  of  the  Palaeozoic  period.  The  ancient  Spirifers  and 
Leptaenids  lingered  on  until  the  Jurassic  period,  and  then  dis- 


Fig.     174.— Jurassic    . 

urchin   (Cidaris  florigem 
wt«,  §),  Corallian. 


THE  JURASSIC  PERIOD  313 

appeared.  On  the  other  hand,  the  genera  Rhynchonella  and 
Terebratula,  which  occupied  a  subordinate  place  in  earlier  ages, 
now  became  the  chief  representatives  of  the  brachiopods.  They 
abounded  throughout  Mesozoic  time,  but  they  have  gradually 
diminished  in  number  since  then,  and  at  the  present  day  each 
genus  survives  only  in  a  small  number  of  species.  With  the 
decay  of  the  brachiopods,  the  other  divisions  of  the  mollusca 
proportionately  advanced.  The  lamellibranchs  attained  a  great 
development  in  Mesozoic  time,  some  characteristic  genera  being 
Gervillia,  Exogyra,  Lima,  Ostrea,  Pecten,  Pinna,  Astarte,  Hip- 


Fig.  175. —  Jurassic  Lamellibranchs.  (a)  Trigonia  monilifera  (§), 
Kimmeridge  Clay;  (&)  Plicatula  spinosa  (§),  Middle  Lias;  (c)  Gry- 
phwa  arcuata  (incurva)  (|),  Lower  Lias. 

popodium,  Trigonia  (Fig.  175).  Some  of  the  oysters  were  par- 
ticularly abundant,  Gryphcea,  for  instance,  being  so  plentiful  in 
some  bands  of  limestone  as  to  give  the  name  of  "  Gryphite 
limestone"  to  them.  But  undoubtedly  the  distinctive  feature 
of  the  molluscan  fauna  of  Mesozoic  time  was  the  great  develop- 
ment of  the  cephalopods.  The  chambered  division  was  repre- 
sented by  an  extraordinary  variety  of  Ammonites  (Fig.  176), 
and  the  cuttle-fishes  by  many  species  of  Belemnite  (Fig.  177). 
The  ammonites  have  been  made  use  of  to  mark  off  the  formations 
into  distinct  zones;  for,  as  a  rule,  the  vertical  range  of  each 


314 


GEOLOGY 


species  is  comparatively  small.  The  band  of  strata  characterised 
by  a  particular  species  of  ammonite  is  called  the  zone  of  that 
species,  e.g.  Zone  of  Amm.  planorbis,  which  is  the  lowest  zone 
of  the  Lower  Lias. 


Fig.  176. —  Jurassic  Ammonites,  (a)  Ammonites  striatus  (i),  Middle 
Lias;  (6)  A.  communis  (§),  Upper  Lias;  (c)  A.  cordatus  (|), 
Lower  Calcareous  Grit;  (d)  A.  Jason  (J),  Oxford  Clay. 

Another  striking  contrast  is  presented  by  the  Jurassic  crus- 
tacea  when  compared  with  those  of  the  Palaeozoic  ages.  The 
ancient  order  of  trilobites,  so  abundant  in  the  seas  of  the  older 


Fig.  177. —  Jurassic  Belemnite  (B.  hastatus,  natural  size),  Middle  Oolite. 

time,  had  now  entirely  disappeared ;  the  eurypterids,  which  took 
their  place  upon  the  scene  as  the  trilobites  were  on  the  wane, 
had  likewise  vanished.  In  their  stead  there  now  came  abundant 


THE  JURASSIC  PERIOD  315 

ten-footed  Crustacea,  including  both  long-tailed  forms  —  the 
ancestors  of  our  lobsters,  prawns,  shrimps,  and  cray-fish —  and 
short-tailed  forms  that  heralded  the  coming  of  our  living  crabs 
(Fig.  178).  Among  the  Jurassic  strata  there  occasionally  oc- 
cur thin  bands,  which  have  received  the  name  of  "  insect-beds  " 


Fig.  178. —  Jurassic  Crustacean  (Scapheus  ancylochelis) . 

from  the  numerous  insect-remains  which  they  contain.  The 
neuroptera  are  most  frequent,  but  orthoptera  and  coleoptera  also 
occur.  Among  these  remains  are  forms  of  dragon-fly,  May-fly, 
grasshopper,  and  cockroach.  The  wing-cases  of  beetles  also 
are  not  uncommon;  and  there  has  been  found  the  wing  of  a 
butterfly  —  the  oldest  example  of  a  lepidopterous  insect  yet 
known. 


Fig.  179. —  Jurassic  Fish   (Pholidophorus  Bechei,  J),  Lower  Lias. 

Fishes  abounded  in  the  waters  of  the  Jurassic  time.  Those  of 
which  the  remains  have  been  preserved  are  chiefly  small  ganoids 
(Plwlidopliorus,  Dapedius,  Lepidotus,  Pycnodm,  Fig.  179), 
with  no  representatives  of  the  huge  bone-cased  placoderms  of 
earlier  time.  There  were  likewise  various  tribes  of  sharks  and 
rays  (Hybodus,  Acrodus,  Squaloraia). 


316  GEOLOGY 

But  taking  the  Jurassic  fauna  as  a  whole,  undoubtedly  its 
most  striking  character  was  given  by  the  extraordinary  develop- 
ment of  its  reptiles.  So  remarkably  varied  was  the  reptilian 
life  throughout  the  Mesozoic  period  that  this  part  of  the  earth's 
history  has  been  called  the  "  Age  of  Reptiles."  There  were  forms 
which  haunted  the  sea,  others  that  frequented  the  rivers,  some 
that  lived  on  the  land,  some  that  flew  through  the  air.  Never 
before  or  since  has  there  been  such  a  profusion  of  reptilian 
types.  Some  of  these  are  still  represented  at  the  present  time. 
The  Jurassic  Teleosaurus  and  Steneosaurus,  for  example,  have 
their  counterparts  now  in  the  living  crocodile  and  alligator. 
The  modern  turtles,  too,  are  descendants  of  those  which  lived  in 
Jurassic  times.  But  it  is  the  long-extinct  types  that  fill  us 
with  astonishment.  One  of  the  most  abundant  of  them  is  that 
of  the  enaliosaurs  or  sea-lizards,  of  which  the  two  leading  forms 
were  the  Ichthyosaurus  and  Plesiosaurus.  The  former  creature 


Fig.  180. —  Jurassic  Sea-lizard  (Ichthyosaurus  communis,  ^),  Lias. 

(Fig.  180),  occasionally  more  than  24  feet  long,  somewhat  re- 
sembled a  whale  in  shape  and  bulk,  its  head  being  joined  by  no 
distinct  neck  to  the  body,  which  tapered  into  a  long  tail.  It 
swam  by  means  of  two  pairs  of  strong  paddles,  and  probably 
steered  itself  by  a  fin  on  the  tail.  Its  eyes  were  large,  and  had 
a  ring  of  bony  plates  round  the  eyeball,  which  remain  distinct 
in  the  fossil  state.  Its  jaws  were  arnied  with  numerous  strong 
pointed  teeth,  not  set  in  distinct  sockets.  This  reptile  probably 
lived  chiefly  in  the  sea,  feeding  there  upon  the  abundant  ganoid 
fishes  which  its  huge  protected  eyes  enabled  it  to  track  even  into 
the  deeper  water.  But  it  no  doubt  also  sought  the  land,  and 
was  able  to  waddle  along  the  shore  or  to  lie  there  basking  in 
the  sunshine.  The  Plesiosaurus,  in  many  respects  like  the 
Ichthyosaurus,  was  distinguished  by  its  proportionately  shorter 
tail,  longer  neck,  smaller  head,  larger  paddles,  and  the  insertion 
of  the  teeth  in  distinct  sockets.  It  probably  haunted  the  la- 
goons, rivers,  and  shallow  seas  of  the  time.  Its  long  swan-like 


THE  JURASSIC  PERIOD  317 

neck  enabled  it  to  lie  at  the  bottom  and  raise  its  head  to  the 
surface  to  breathe,  or/ when  at  the  surface,  to  send  down  its 
powerful  jaws  and  catch  its  prey  at  the  bottom. 

Still  more  extraordinary  were  the  Pterosaurs  or  flying  rep- 
tiles—  strange  bat-like  creatures  with  disproportionately  large 
heads,  and  large  eyes  like  those  of  the  Ichthyosaurus.  The  outer- 
most finger  of  each  forefoot  was  prolonged  to  a  great  length, 
and  supported  a  membrane  with  which  the  animals  could  fly. 
The  bones  were  hollow  and  filled  with  air  like  those  of  birds. 
Various  forms  of  these  winged  lizards  are  found  in  the  Jurassic 
rocks,  the  most  typical  being  Pterodactylus  and  Scaphognatlius 
(Fig.  181)  ;  others  are  Dimorphodon,  Rhamphorhynchtis, 
Eliamplwceplialus,  and  Dorygnathus. 


Fig.  181. — Jurassic  Pterosaur,  or  flying  reptile  (Scaphognathus  crassiroS' 
tris),  Middle  Oolite. 

The  Deinosaurs,  which  have  already  been  noticed  as  having 
appeared  in  Triassic  time,  attained  a  far  greater  development  in 
the  Jurassic  period.  These  hugest  land-reptiles  now  reached 
their  maximum  in  size  and  variety.  One  of  their  genera, 
Megalosaurus,  is  believed  to  have  been  25  feet  long,  and  to  have 
walked  on  its  massive  hind  legs  along  the  margins  of  the  shallow 
waters  in  search  of  the  smaller  animals  on  which  it  preyed. 
Another  form,  Ceteosaurus,  which  may  have  been  as  much  as  50 
feet  from  the  snout  to  the  tip  of  the  tail,  and  stood  some  10 
feet  high,  fed  on  the  vegetation  that  shaded  the  rivers  and  la- 
goons where  it  lived.  Still  more  gigantic  were  some  deinosaurs, 


318 


GEOLOGY 


of  which  the  remains  have  been  found  in  the  Jurassic  rocks  of 
North  America.  Brontosaurus,  about  50 'feet  or  more  in  length 
with  a  short  body,  long  neck  and  tail,  and  small  head,  had  enor- 
mous feet,  each  of  which  made  an  imprint  measuring  about  a 
square  yard  in  area.  Stegosaurus,  another  sluggish  deinosaur, 
was  protected  by  numerous  huge  plates  and  spines  of  bone  on  its 


Fig.  182. — Jurassic  Bird   (Arcliccoptcryx  macroura,  about  |),  Solenhofen 
Limestone  Middle  Jurassic. 

back,  some  of  the  latter  more  than  3  feet  long.  The  largest  of 
all  these  monsters,  and,  so  far  as  yet  known,  the  most  colossal 
animal  that  ever  walked  on  the  earth,  was  the  Atlaniosaurus, 
which  is  believed  to  have  been  not  much  less  than  100  feet  in 
length,  and  30  feet  or  more  in  height. 

In  another  respect,  the  fauna  of  the  Jurassic  period  stands 
out  from  those  that  preceded  it ;  it  contained  the  earliest  known 
birds.  These  interesting  prototypes  differed  much  from  modern 
birds,  more  particularly  in  the  possession  of  certain_  peculiarities  j 


THE  JURASSIC  PERIOD 


319 


of  structure  that  linked  them  with  reptiles.  They  had  teeth 
in  their  jaws,  and  some  of  them  carried  long  lizard-like  tails, 
each  vertebra  of  which  bore  a  pair  of  quill-feathers.  The  best 
known  genus  is  Archceopteryx  (Fig.  182),  found  in  the  litho- 
graphic limestone  of  Solenhofen. 

Marsupials,  which,  so  far  as  yet  known,  made  their  appear- 
ance in  Triassic  time,  continued  to  be  the  only  representatives 


Fig.     183. —  Jurassic     Marsupial     (Phascolotherium     Bucklandi).     (a) 
Teeth  magnified;   (6)   Jaw,  natural  size. 

of  the  mammalia  during  the  Jurassic  period,  at  least  no  other 
types  have  yet  been  discovered  among  the  fossils.  Lower  jaws 
and  detached  teeth  (Fig.  183)  have  been  obtained  from  two  dis- 
tinct platforms  in  England  —  the  Stonesfield  Slate  and  Purbeck 
beds  —  and  have  been  referred  to  a  number  of  genera  which 
find  their  nearest  modern  representatives  in  the  Australian 
bandicoots  and  in  the  American  opossums  (Phascolotherium^ 
Stereognatlms,  SpalacotJienum,  Plagiaulax) . 

The  Jurassic  system  has  been  arranged  in  the  following  sub- 
divisions : — 


as 

si 


I. 


8.  Purbeckian 


7.  Portlandian. 


Am- 


Am- 


o  L 


Upper   fresh-water   beds    (Purbeck). 

Middle  marine  beds 

Lower  fresh-water  beds 

Limestones  and  calcareous  freestones  (Portland 
Stone)  ;  Cerithium  portlandicum,  Ammonites 
giganteus,  Trigonia  gibbosa. 

Sandstones  and  marls    (Portland  Sand)  ; 

monites  biplex,  Exogyra  bruntrutana. 

6.  Kimmeridgianf  Dark  shales  and  clays  (Kimmeridge  Clay) 
\      monites  decipiens,  Exogyra  virgula. 

Coral  rag  (limestone  with  corals),  clays,  and 
calcareous  grits;  Thamnastrcea,  Isastraea, 
Cidaris  ftorigemma,  Ammonites  cordatus  (Fig. 
176). 

Blue  and  brown  clay  (Oxford  Clay)  ;  Ammon- 
ites Jason  (Fig.  176). 

Calcareous  sandstone  (Kellaways  Rock  —  Cal- 
lovian)  ;  Ammonites  calloviensis. 


Corallian 


4.  Oxfordian 


320 


GEOLOGY 


3.  Bathonian    . . 


2.     Bajocian 
(Inferior  Oolite) 


1.  Liassic    


Shelly  limestones,  clays,  and  sands  (Cornbrash, 
Bradford  Clay,  and  Forest  Marble).  Am- 
monites discus. 

Shelly  limestones  (Great  or  Bath  Oolite), 
Stonesfield  Slate;  Ammonites  gracilis. 

Fuller's  Earth. 

Marine  calcareous  freestones  and  grits  (Chel- 
tenham), containing  zones  of  Ammonites  Par- 
kinsoni,  A.  Humphriesianus,  A.  Sowerbyi,  A. 
Hurchisonw ;  represented  in  Yorkshire,  by 
800  feet  or  more  of  estuarine  sandstones, 
shales,  and  limestones,  with  beds  of  coal. 

Sandy  beds  and  clays  (Upper  Lias,  Toarcian)  ; 
Ammonites  communis,  A.  jurensis,  A.  serpcn- 
tinus. 

Limestones,  sands,  clays,  and  ironstones  (Mid- 
dle Lias,  Marlstone)  ;  Ammonites  margarita- 
tus,  A.  spinatus,  A.  Jamesoni. 

Thin  blue  and  brown  limestones,  and  dark 
shales  (Lower  Lias,  Sinemurian  and  Hettan- 
gian)  ;  Ammonites  planorbis,  A.  raricostatus, 
A.  Bucklandi. 


1.  The  LIAS,  so  called  originally  by  the  Somerset  quarrymen 
from  its  marked  arrangement  into  "  layers,"  extends  completely 
across  England  from  Lyme  Kegis  to  Whitby.  It  can  be  divided 
into  three  distinct  sections :  (a)  A  lower  group  of  thin  blue  lime- 
stones and  dark  shales  with  limestone  nodules,  the  limestones 
being  largely  used  for  making  cement.  This  is  one  of  the  chief 
platforms  for  the  reptilian  remains,  entire  skeletons  of  ichthyo- 
saurus, plesiosaurus,  etc.,  having  been  exhumed  at  Lyme  Eegis; 
(&)  Marlstone  or  Middle  Lias  —  hard  argillaceous  or  ferru- 
ginous limestones  which  form  a  low  ridge  or  escarpment  rising 
from  the  plain  of  the  Lower  Lias ;  in  Yorkshire  contains  a  thick 
series  of  beds  of  earthy  carbonate  of  iron,  which  are  extensively 
mined  as  a  source  for  the  manufacture  of  iron;  (c)  Clays  and 
shales  surmounted  by  sandy  beds  (Upper  Lias  Stands).  The 
organic  remains  of  the  Lias  are  abundant  and  well  preserved. 
They  are  chiefly  marine;  but  that  the  rocks  containing  them 
were  deposited  near  land  is  indicated  by  the  numerous  leaves, 
branches,  and  fruits  imbedded  in  them,  and  by  the  various  in- 
sect-remains that  have  been  obtained  from  them.  In  Germany, 
where  the  Lias  is  well  developed  and  presents  a  general  re- 
semblance to  the  English  type,  it  is  known  as  the  Lower  or 


THE  JURASSIC  PERIOD  321 

Black  Jura.  It  is  still  better  shown  in  France,  where  its  three 
stages  attain  in  Lorraine  a  united  thickness  of  more  than  600 
feet.  To  the  south,  however,  in  Provence,  it  reaches  the  great 
thickness  of  2,300  feet. 

2.  The  BAJOCIAN  stage,  so  named  from  Bayeux  in  Normandy, 
where  it  is  well  displayed,  has  long  been  known  in  England 
under  the  name  of  Inferior  Oolite.     It  presents  two  distinct 
types  in  this  country,  being  a  thoroughly  marine  formation  in 
the  southwestern  counties  and  passing  northward  into  a  series 
of  strata  which  were  accumulated  in  an  estuary,  and  which  con- 
tain the  chief  repositories  of  the  Jurassic  flora.     Among  the 
estuarine  beds  of  Yorkshire  a  few  thin  coal-seams  occur,  which 
have  been  worked  to  some  extent.     On  the  continent,  this  divi- 
sion is  characteristically  marine;  it  reaches  its  greatest  develop- 
ment in  Provence,  where  it  is  950  feet  thick.     It  runs  through 
the  Jura  Mountains,  where  it  is  made  up  of  more  than  300  feet 
of  strata,  chiefly  limestone.     In  Germany  the  strata  from  the 
top  of  the  Lias  to  the  base  of  the  Callovian  group  —  that  is,  the 
two  stages  of  Bajocian  and  Bathonian  —  are  classed  together  as 
the  Middle  or  Brown  Jura,  or  Dogger. 

3.  The  BATHONIAN  stage  is  named  from  Bath,  where  its  sub- 
divisions are  admirably  exposed.     At  its  base  is  a  local  argil- 
laceous band  known  as  Fuller's  Earth,  because  long  used  for 
fulling  cloth.     The  chief  member  of  the  stage  in  the  south-west 
of  England  is  the  Great  or  Bath  Oolite,  a  succession  of  lime- 
stones, often  oolitic,  with  clays  and  sands.   The  Stonesfield  Slate 
is  the  name  locally  given  to  some  thin-bedded  limestones  and 
sands  forming  the  lower  part  of  the  Great  Oolite,  and  of  high 
geological  interest  from  having  supplied  among  their  fossils  re- 
mains of  land-plants,  numerous  insects,  bones  of  enaliosaurs  and 
deinosaurs,  and  of  small  marsupials.    The  Great  Oolite  abounds 
in  corals,  and  contains  numerous  genera  of  mollusca,  fishes,  and 
reptiles.     The  Cornbrash  (so  named  from  its  friable  (brashy) 
character,  and  from  its  forming  good  soil  for  corn)  is  one  of  the 
most  persistent  bands  in  the  English  Jurassic  system,  retaining 
its  characters  all  the  way  from  the  south-western  counties  to  near 
the  Humber. 

4.  The  OXFORDIAN  stage,  sometimes  called  the  Middle  or  Ox- 
ford Oolite,  consists  of  a  lower  zone  of  calcareous  sandstone, 
known  as  the  Kellaways  rock  or  Callovian,  from  the  name  of  a 


322  GEOLOGY 

place  in  Wiltshire,  and  of  a  thick  upper  stiff  blue  and  brown 
clay,  called,  from,  the  locality  where  it  is  well  developed,  the 
Oxford  Clay,  and  containing  numerous  ammonites,  belemnites, 
and  oysters,  but  no  corals.  In  Germany,  the  strata  from  the 
base  of  the  Callovian  to  the  top  of  the  Purbeckian  group  are 
known  as  the  Malm  or  White  Jura. 

5.  The  CORALLIAN  stage,  so  named  from  the  corals  with  which 
it  abounds,  is  one  of  the  most  distinctive  in  the  Jurassic  system. 
It  is  traceable  across  the  greater  part  of  England,  over  the 
continent  of  Europe  from  Normandy  to  the   Mediterranean, 
through  the  east  of  France,  and  along  the  whole  length  of  the 
Jura  Mountains  and  the  flank  of  the  Swabian  Alps.     While  it 
was  being  formed,  the  greater  part  of  Europe  lay  beneath  a 
shallow  sea,  the  floor  of  which  was  clustered  over  with  reefs  of 
coral. 

6.  The  KJMMERIDGIAN  group  or  stage  is  typically  displayed 
at  Kimmeridge  on  the  coast  of  Dorsetshire,  whence  its  name. 
It  there  consists  of  dark  shales,  some  of  which  are  so  highly 
bituminous   as  to   burn  readily,  and  which  will  probably  be 
eventually  of  commercial  value  as  a  source  for  the  distillation 
of  mineral  oil.     This  group  of  strata  has  yielded  a  larger  num- 
ber of  reptilian  genera  and   species   than   any   other  in  the 
Mesozoic  systems  of  Britain  —  plesiosaurs,  ichthyosaurs,  ptero- 
saurs, deinosaurs,  turtles,  and  crocodiles.     It  is  well  developed 
in  France  and  Germany. 

7.  The  PORTLANDIAN  stage,  so  called  from  the  Isle  of  Port- 
land where  it  is  well  seen,  consists  of  a  lower  set  of  sandy  beds 
(Portland  Sand),  and  a  higher  and  thicker  series  of  limestones 
and  calcareous  freestones,  some  of  the  beds  containing  abundant 
nodules  and  layers  of  flint.     These  rocks  are  prolonged  into 
France  near  Boulogne-sur-Mer. 

8.  The  PURBECKIAN  group  or  stage  is  best  seen  in  the  Isle  of 
Purbeck,  hence  its  name.     It  lies  on  an  upraised  surface  of 
Portlandian  beds,  showing  that  after  the  deposition  of  these 
beds  there  was  some  disturbance  of  the  sea-bed,  portions  of 
which  were  uplifted  partly  into  land  and  partly  into  shallow 
brackish  and  fresh  waters.     The  Purbeck  beds  are  subdivided 
into  three  sub-stages :  the  lowest  consisting  of  fresh-water  lime- 
stones, with  layers  of  ancient  soil  ("  dirt-beds  "),  in  which  the 
stumps  of  cycadaceous  trees  still  stand  in  the  positions  in  which 


THE  JURASSIC  PERIOD  323 

they  grew  (Fig.  171) ;  the  middle  sub-stage  contains  oysters  and 
other  marine  shells  which  prove  that  owing  to  subsidence  the 
area  sank  under  the  sea;  while  in  the  higher  subdivision  fresh- 
water fossils  reappear.  Among  the  more  interesting  organisms 
yielded  by  the  Purbeck  beds  are  the  remains  of  numerous  insects 
and  of  the  marsupials  already  referred  to,  which  chiefly  occur  as 
lower  jaws  in  a  stratum  about  5  inches  thick.  When  the  bodies  of 
dead  animals  float  out  to  sea  the  first  bones  likely  to  drop  out 
of  the  decomposing  carcases  are  the  lower  jaws;  hence  the 
greater  frequency  of  these  bones  in  the  fossil  state.  Strata  be- 
longing to  the  Purbeckian  stage  and  including  red  and  green 
marls,  with  dolomite  and  gypsum,  are  found  in  north-western 
Germany,  showing  in  that  region  also  the  elevation  of  the  floor 
of  the  Jurassic  sea  into  detached  basins. 

In  India,  a  mass  of  strata  6,300  feet  thick  is  found  in  Cutch, 
and  from  its  fossils  is  believed  to  represent  the  European  Juras- 
sic system  from  the  Bajocian  up  to  the  top  of  the  Portlandian 
stage.  In  Australia  and  New  Zealand,  recognisable  Jurassic 
fossils  have  also  been  found,  showing  the  extension  of  the 
Jurassic  system  even  to  the  Antipodes.  In  North  America, 
Jurassic  rocks  are  not  largely  developed;  but  in  Colorado  they 
have  yielded  an  abundant  series  of  organic  remains,  includ- 
ing fishes,  tortoises,  pterosaurs,  deinosaurs,  crocodiles,  and  mar- 
supials. 


324  GEOLOGY 


CHAPTER  XXIV. 

THE   CRETACEOUS   PERIOD. 

THE  CEETACEOUS  system  received  its  name  in  Western 
Europe,  because  in  England  and  in  Northern  France  its 
most  conspicuous  member  is  a  thick  mass  of  white  chalk 
(Latin,  Creta).  It  covers  a  far  more  extensive  area  of  the 
surface  of  this  continent  than  any  of  the  preceding  systems. 
Its  western  extremity  reaches  to  the  north  of  Ireland  and  the 
Western  Islands  of  Scotland.  It  covers  a  large  part  of  the 
east  and  south  of  England,  stretching  thence  into  France,  where 
it  forms  a  broad  band,  encircling  the  Tertiary  basin  of  Paris.  It 
sweeps  across  Belgium  into  Westphalia,  underlies  the  vast  plain 
of  Northern  Germany  and  Denmark,  whence  it  is  prolonged  into 
Southern  Russia,  where  it  overspreads  many  thousands  of  square 
miles.  It  flanks  most  of  the  principal  mountain-chains  of  Eu- 
rope—  the  Pyrenees,  Alps,  Apennines,  and  Carpathians.  It 
spreads  far  and  wide  -over  the  basin  of  the  Mediterranean  Sea, 
extending  across  vast  tracts  of  Northern  Africa,  and  from  the 
Adriatic  athwart  Greece  and  Turkey  into  Asia  Minor,  whence  it 
is  prolonged  through  the  Asiatic  continent. 

As  most  of  the  rocks  of  the  system  are  of  marine  origin,  we 
at  once  perceive  how  entirely  different  the  Cretaceous  geography 
must  have  been  from  that  of  the  present  day,  and  to  what  a  great 
extent  the  existing  land  of  the  Old  World  lay  then  below  the  sea. 
But  in  tracing  out  the  distribution  of  the  rocks,  geologists  have 
found  that  the  Cretaceous  sea  did  not  sweep  continuously  across 
Europe.  On  the  contrary,  as  they  have  ascertained,  the  old 
northern  land  still  rose  over  the  site  of  Northern  Britain  and 
Scandinavia,  while  to  the  south  of  it  a  wide  depression  extended 
across  the  area  of  Southern  Britain,  Northern  France,  Belgium, 
and  the  North  German  plain,  eastwards  to  Bohemia  and  Silesia. 
This  vast  northern  basin  was  the  theatre  of  a  remarkable  suc- 
cession of  geological  revolutions.  While  its  eastern  portions, 


THE  CRETACEOUS  PERIOD  325 

during  the  earlier  part  of  the  Cretaceous  period,  were  submerged 
under  the  sea,  its  western  tracts  were  the  site  of  the  delta  of  a 
great  river,  probably  descending  from  the  land  that  still  lay 
massed  towards  the  north.  During  the  later  ages  of  the  period, 
the  whole  of  this  area  formed  a  broad  and  long  gulf  or  inlet, 
the  southern  margin  of  which  seems  to  have  been  defined  by  the 
ridge  of  old  rocks  that  runs  from  the  headlands  of  Brittany 
through  Central  France,  the  Black  Forest,  and  the  high  grounds 
of  Bohemia.  South  of  that  ridge  lay  the  open  ocean  which 
extended  all  over  Southern  Europe  and  the  north  of  Africa,  and 
spread  eastwards  into  Asia. 

Bearing  in  mind  this  peculiar  disposition  of  sea  and  land, 
we  can  understand  why  the  development  of  the  Cretaceous  sys- 
tem, alike  in  regard  to  its  deposits  and  its  fossils,  should  be  so 
different  in  the  area  of  the  northern  basin  from  that  of  the 
southern  regions.  In  the  one  case  we  meet  with  the  local  and 
changing  accumulations  of  a  comparatively  shallow  and  some- 
what isolated  portion  of  the  sea-bed,  wherein  are  mingled  abun- 
dant traces  of  the  proximity  of  land.  In  the  other  we  are 
presented  with  evidence  of  a  wide  open  sea,  where  the  same 
kinds  of  deposits  and  the  same  forms  of  marine  life  extend 
with  little  change  over  vast  distances.  Obviously,  it  is  not 
the  local  type  of  the  northern  basin,  but  the  more  general  and 
widespread  type  of  Southern  Europe  that  should  be  taken  for 
the  distinctive  characteristics  of  the  Cretaceous  system.  But 
the  northern  basin  was  the  first  to  be  systematically  explored, 
and  is  still  the  best  known,  and  hence  its  features  have  not  un- 
naturally usurped  the  place  of  importance  which  ought  properly 
to  be  assigned  to  the  other  wider  area. 

Eegarding  the  period  as  a  whole,  let  us  first  consider  the 
general  character  of  its  distinguishing  flora  and  fauna,  and  then 
pass  on  to  trace  the  history  of  the  period  as  revealed  by  the 
succession  of  strata.  The  plants  of  the  Cretaceous  system  show 
that  the  vegetable  kingdom  had  now  made  a  most  important 
advance  in  organisation.  In  the  lower  half  of  the  system  the 
fossil  plants  yet  found  are  on  the  whole  like  those  of  the  Jurassic 
rocks  —  that  is,  they  include  some  of  the  same  genera  of  ferns, 
cycads,  and  conifers  which  these  rocks  contained.  But  already 
the  ancestors  of  our  common  trees  and  flowering  plants  must 
have  made  their  appearance,  for  in  the  upper  half  of  the  system 


326 


GEOLOGY 


their  remains  occur  in  abundance.  This  earliest  dicotyledonous 
flora  numbered  among  its  members  species  of  maple,  alder, 
aralia,  poplar,  myrica,  oak,  fig,  walnut,  beech,  plane,  sassafras, 
laurel,  cinnamon,  ivy,  dogwood,  magnolia,  gum-tree,  ilex,  buck- 
thorn, cassia,  credneria,  and  others.  The  modern  aspect  of  this 
assemblage  of  plants  is  in  striking  contrast  to  the  more  antique 
look  of  all  the  older  floras.  There  were  likewise  species  of  pine 
(Pinus),  Calif  ornian  pine  (Sequoia),  juniper,  and  other  coni- 


Fig.  184. — Cretaceous  Plants,  (a)  Quercus  rinkiana  (§)  ;  (6)  Cinna- 
momum  sezannense  (§)  ;  (c)  Ficus  atavina  (§)  ;  (a)  Sassafras  re- 
curvata  (§)  ;  (e)  Juglans  arctica  (i). 

fers,  various  cycads,  forms  of  screw-pine  (Pandanus),  palms 
(Sabal),  and  numerous  ferns  (Gleichenia,  Asplenium,  etc.). 
This  flora  spread  over  the  land,  surrounding  the  northern  Creta- 
ceous basin,  and  extended  northwards  even  as  far  as  North 
Greenland,  from  which  nearly  200  species  of  Cretaceous  plants 
have  been  obtained.  The  inference  may  be  deduced  that  the 


THE  CRETACEOUS  PERIOD 


327 


climate  of  the  globe  must  then  have  been  much  warmer  than  at 
present.  The  luxuriant  vegetation  disinterred  from  the  Creta- 
ceous rocks  of  North  Greenland  includes  more  than  forty  kinds 
of  ferns,  besides  laurels,  figs,  magnolias,  and  other  plants,  which 


Fig.     185. —  Cretaceous     Foraminifera.      (a)      Textularia     ~baudouiniana 
(-T)  5  (6)  QloUgerina,  cretacea  (V1);  (c)  Rotalina  voltziana  (^). 

show  that,  though  the  winters  were  no  doubt  dark,  they  must 
have  been  extremely  mild.  There  could  have  been  no  perpetual 
frost  and  snow  in  these  Arctic  latitudes  in  Cretaceous  times. 

Foraminifera  abound  in  some  of  the  Cretaceous  limestones, 
indeed,  in  some  places  they  form  almost  the  only  constituent  of 
these  rocks.  They  are  plentiful  in  the 
white  chalk  of  England,  France,  and  Bel- 
gium, one  of  the  more  frequent  genera 
being  Gloligerina  (Fig.  185)  which  still 
lives  in  enormous  numbers  in  the  Atlan- 
tic, and  forms  at  the  bottom  of  that 
ocean  a  gray  ooze  not  unlike  chalk  (Fig. 
33).  Sponges  lived  in  great  numbers  in 
the  Cretaceous  sea.  Their  minute  sili- 
ceous spicules  are  abundant  in  the  Chalk, 
and  even  entire  sponges  enveloped  in  flint 
are  not  uncommon  (Ventriculites,  Fig. 
186).  Sea-urchins  are  among  the  most 
familiar  fossils  of  the  Chalk,  and  must 
have  lived  in  great  numbers  on  the  Cre- 
taceous sea-bottom.  Some  of  their 
genera  are  still  living,  and  have  been 
recent  years  from  great  depths  in  the  ocean 


Fig.  186. —  Cretaceous 
Sponge  ( Ventricu- 
lites decurrens,  J). 


dredged    up    in 
Among  the  more 

characteristic  Cretaceous  types  are  Anancliytes,  Holaster,  Mi- 
craster,  and  Echinoconus  (Fig.  187).  The  brachiopods  were 
still  represented  chiefly  by  the  ancient  genera  Terebmtula  and 


328 


GEOLOGY 


Rliynclionella.  Lamellibranchs  abounded,  especially  the  genera 
Ostrea,  Exogyra,  Inoceramus  (Fig.  188),  Lima,  Pecten,  and 
the  various  forms  of  Hippuritids.  These  last  (Hippurites, 
Radiolites,  Caprina,  etc.,  Fig.  189)  are  specially  characteristic, 
being,  so  far  as  we  know,  confined  to  the  Cretaceous  system; 
hence  their  occurrence  serves  to  indicate  the  Cretaceous  age  of 
the  rock  containing  them.  They  have  been  imbedded  in  such 


Fig.  187. —  Cretaceous  Sea-urchins,  (a)  Echinoconus  conicus,  §  (  = 
Galerites  albo-galerus) ,  under  surface  and  side  view;  (6)  Ananchytes 
ovatus  (J),  side  view  and  under  surface;  (c)  Micraster  cor-arguinum 
(5),  upper  and  under  surface. 

numbers  in  the  limestones  of  the  south  of  Europe  as  to  give  the 
name  of  hippurite-limestone  to  these  rocks.  They  are  compara- 
tively infrequent  in  the  strata  of  the  northern  Cretaceous  basin. 
Probably  the  most  distinctive  feature  in  the  molluscan  life  of 
the  Cretaceous  seas  was  the  extraordinary  variety  in  the  devel- 


THE  CRETACEOUS  PERIOD 


329 


opment  of  the  cephalopods.  This  is  all  the  more  remarkable 
from  the  fact  that  before  the  next  geological  period  the  great 
majority  of  these  types  appear  to  have  become  extinct.  The 
ammonites  and  belemnites,  which  played  so  important  a  part 


Fig.    188. —  Cretaceous    Lamellibranchs.     (a)     Trigonia    aliformis    (1)  ; 
(6)  Inoceramus  sulcatus  (i)  ;   (c)   Nucula  bivirgata  (natural  size). 

in  the  fauna  of  Mesozoic  time,  died  out  about  the  close  of  that 
long  succession  of  periods.  At  least  in  Europe,  while  their  re- 
mains continue  to  present  themselves  up  to  the  top  of  the  Greta- 


Fig.  189. —  Cretaceous  Lamellibranchs  (Hippurites).  (a)  Radiolites 
acuticostata  (£)  ;  (6)  Hippurites  toucasiana  (S)  ;  (c)  Caprina  Aguil- 
loni  (J)  ;  (d)  Caprotina  toucasianus  (fe). 

ceous  system,  they  disappear  entirely  from  the  overlying  strata. 
It  is  curious  to  observe  that  while  these  important  tribes  were 


330 


GEOLOGY 


about  to  vanish,  other  cephalopods  of  new  and  varied  types 
nourished  contemporaneously  with  them.  Never  before  or  since, 
indeed,  have  the  cephalopodan  types  been  so  manifold  (Fig. 


Fig.  190. — Cretaceous  Cephalopods.  (a)  B acuities  anceps  (J)  ;  (&)  Pty- 
choceras  emericianus  (\)  ;  (c)  Toxoceras  Utulerculatus  (I)  ;  (d!) 
Hamites  rotundus  (J)  ;  (e)  Anculoceras  renauxianus  (TV)  ;  (f)  Sca- 
phites  cequalis  (§)  ;  (g)  Crioceras  villiersianus  (J)  ;  (h)  Helicoceras 
annulatus;  (i)  Ammonites  inflatus  (k)  ',  (k)  Turrilites  catenatus  (J). 

190).  For  instance  Baculites  is  a  straight  chambered  shell  re- 
minding us  of  the  ancient  OrtJioceras.  In  Toxoceras  the  shell 
is  bent  into  the  form  of  a  bow.  In  Hamites  it  is  long,  tapering, 
and  curved  upon  itself  like  a  hook.  In  Ancyloceras  it  is  coiled 


THE  CRETACEOUS  PERIOD  331 

at  the  posterior  end,  the  other  being  bent  back  upon  itself ;  while 
in  Scaphites  the  coils  are  adherent.  In  Ptychoceras  the  shell  is 
long,  tapering,  and  bent  once  back  on  itself,  the  two  portions  be- 
ing in  contact.  In  Crioceras  it  is  coiled,  and  the  coils  are  not 
adherent,  as  they  are  in  the  ammonites.  In  Helicoceras  the 
shell  is  coiled  spirally,  the  coils  remaining  free,  while  in  Tur- 
rilites  they  are  adherent. 

The  fishes  of  the  Cretaceous  period  are  chiefly  known  by 
teeth  belonging  to  various  genera  of  sharks  (Otodus,  Lamna, 
Oxyrhina) .  But  they  also  include  the  earliest  known  representa- 
tives of  the  modern  osseous  or  teleostean  fishes,  such  as  the 
herring,  salmon,  and  cod  (Osmeroides,  Enchodus,  Beryx,  Fig. 
191,  Syllcemus,  etc.). 


Fig.   191. —  Cretaceous  Fish    (Beryx  lewesiensis,   I). 

Already  reptilian  life  seems  to  have  been  on  the  decline,  at 
least  there  is  much  less  variety  and  abundance  of  it  in  the 
Cretaceous  system  than  in  that  which  immediately  preceded  it. 
Turtles  and  tortoises  continued  to  haunt  the  low  shores  of  the 
time.  Ichthyosaurs,  plesiosaurs,  pterosaurs,  and  deinosaurs  still 
lived,  but  in  diminishing  numbers,  and  they  are  not  known  to 
have  survived  the  Cretaceous  period.  One  of  the  most  remarkable 
of  the  deinosaurs,  and  interesting  from  being  one  of  the  last 
of  its  race,  is  that  known  as  Iguanodon  (Fig.  192).  Only  scat- 
tered teeth  and  bones  of  this  animal  were  known,  until  a  few 
years  ago  the  fortunate  discovery  of  a  number  of  entire  skeletons 
in  Belgium  enabled  its  structure  to  be  almost  completely  made 
known,  and  threw  much  fresh  light  on  the  osteology  of  the 
deinosaurs.  It  was  a  herbivorous  and  probably  amphibious 
creature,  able,  no  doubt,  to  walk  along  the  shores,  with  an  un- 


332  GEOLOGY 

wieldy  gait,  on  its  long  hind  legs,  and  balancing  itself  by  its 
strong  massive  tail,  which  was  doubtless  a  powerful  instrument 
of  propulsion  through  the  water.  Its  extraordinary  fore  legs, 
with  the  strong  spurs  on  the  digits,  must  have  been  formidable 
weapons  of  defence  against  its  carnivorous  contemporaries.  An- 
other gigantic  reptile,  the  Mosasaurus,  believed  to  have  been  75 
feet  long,  was  furnished  with  fin-like  paddles  for  swimming. 
Several  kinds  of  crocodiles  have  also  been  disinterred  from 
Cretaceous  rocks  in  Europe. 


Fig.  192. —  Cretaceous  deinosaur    (Iguanodon,  about 

Still  more  remarkable  is  the  assemblage  of  remains  of  animal 
life  exhumed  from  corresponding  rocks  in  the  Western  Terri- 
tories of  North  America.  Among  these  the  Discosaurus  was  a 
snake-like  animal  some  40  feet  long,  with  a  swan-like  neck  sup- 
porting a  slim  head  which  it  could  raise  20  feet  out  of  the  water, 
or  dart  to  the  bottom  and  catch  its  prey.  The  pythonomorphs 
or  sea-serpents  were  especially  numerous. 

The  remains  of  true  birds  have  been  obtained  from  the  Cre- 
taceous rocks  both  of  Europe  and  North  America.  They  are  re- 
lated to  the  living  ostrich,  but  some  of  them  were  furnished  with 
teeth  set  in  a  continuous  groove  (Hesperornis),  others  had  large 
teeth  in  distinct  sockets  (Ichthyornis). 

The  following  are  the  principal  subdivisions  of  the  Cretaceous 
system  in  Europe  in  descending  order.  The  stages  are  based 
upon  more  or  less  well-marked  fossil  evidence,  but  they  are 


THE  CRETACEOUS  PERIOD 


333 


Danian 


Senonian , 


I 


Turonian 


Cenomanian 


also  for  the  most  part  to  be  distinguished  by  lithological  char- 
acters : — 

Pisolitic  limestone  of  Paris  basin;  Chalk  of 
Hainault,  Ciply,  Maestricht,  Faxoe  in  Denmark, 
and  the  south  of  Sweden;  absent  in  England 
(Belemnitella,  Baculites). 

Chalk-with-flints  of  Norwich,  Brighton,  Flam- 
borough  Head,  and  Dover,  north  of  France 
(Belemnitella,  Harsupites,  Micraster)  ;  sand- 
stones of  Westphalia  and  Saxony. 

Chalk-without-flints  of  Dover  and  north  of  France 
(Holaster  planus,  Terebratulina  gracilis,  Inoce- 
ramus  labiatus)  ;  sandstones,  limestones,  and 
marls  of  Saxony  and  Bohemia ;  Hippurite  lime- 
stone of  Southern  France  and  Mediterranean 
basin. 

Gray  Chalk  of  Folkestone  (Holaster  subglobosus, 
Belemnitella  plena],  Chalk-Marl,  red  chalk  of 
Hunstanton,  Glauconitic  Marl  and  Upper  Green- 
sand  (Ammonites  rostratus,  Pecten  asper)  ; 
Chalk  of  Rouen ;  earthy  limestones  and  marls  in 
Hanover  replaced  southwards  by  plant-bearing 
sandstones,  clays,  and  thin  coal-seams ;  Hippurite 
limestones  of  Southern  Europe. 

Gault  (Ammonites  cristatus,  A.  denarius,  A. 
auritus) . 

In  Southern  England  a  fluviatile  (partly  marine) 
succession  of  sands  and  clays  (Wealden),  sur- 
mounted by  sands,  clays,  and  limestones  (Lower 
Greensand),  in  Northern  England  a  series  of 
clays  and  limestones,  with  marine  fossils  (upper 
part  of  Speeton  clay)  ;  limestones  and  marls  of 
Neuchatel;  compact  crystalline  limestones  in 
Provence  (Ammonites  Dehayesi,  A.  nisus  in 
upper  division;  abundant  Ancyloceras  with 
Pecten  cinctus  in  middle ;  Ammonites  noricus,  A. 
astieramus,  Ostrea  Couloni  in  lower). 

It  will  be  remembered  that  towards  the  close  of  the  Jurassic 
period  the  floor  of  the  sea  in  the  western  part  of  the  European 
area  was  gently  raised,  some  of  the  younger  Jurassic  marine 
limestones  being  ridged  up  into  islets  or  low  land,  with  lakes  or 
estuaries  in  which  the  Purbeck  beds  were  deposited.  This  ter- 
restrial condition  of  the  geography  was  maintained  and  ex- 
tended in  the  same  region  during  the  early  part  of  the  Cretaceous 
period.  The  geological  history  of  Europe  as  revealed  by  the 


Albian 


Neocomian 


334  GEOLOGY 

various  subdivisions  in  the  foregoing  Table  may  be  briefly  given, 
NEOCOMIAN  (from  Neocomum,  the  old  Latin  name  of  Neu- 
chatel  in  Switzerland).  This  stage  in  the  south  of  England 
and  thence  eastwards  across  Hanover  consists  of  a  mass  of  sand 
and  clay  sometimes  1,800  feet  thick,  representing  the  delta  of  a 
river.  Only  a  portion  of  this  delta  remains,  but  as  it  extends  in 
an  east  and  west  direction  for  at  least  200,  and  from  north  to 
south  for  perhaps  100  miles,  its  total  area  may  have  been  20,000 
square  miles,  indicating  a  large  river  comparable  with  the 
Quorra  of  the  present  day.  This  stream  not  improbably  de- 
scended from  the  north  or  north-west.  It  carried  down  the 
drifted  vegetation  of  the  land,  with  occasional  carcases  of  the 
iguanodons  and  other  terrestrial  or  amphibious  creatures  of  the 
time.  From  theii  great  development  in  the  Weald  of  Sussex 
these  delta-deposits  have  been  called  Wealden.  They  attain 
there  a  thickness  of  more  than  1,800  feet  and  consist  of  the 
following  subdivisions  in  descending  order. 

Weald    Clay 1000  feet. 

Hastings   Sand  group,  comprising — 

3.     Tunbridge   Wells   Sand 140  to  380     " 

2.     Wadhurst    Clay 120  to  180     " 

1.     Ashdown    Sand 400  to  500     " 

Beyond  the  area  overspread  by  the  sand  and  mud  of  the 
delta,  the  ordinary  marine  sediments  accumulated,  with  their 
characteristic  organic  remains.  We  find  these  sediments  in 
Yorkshire  (upper  part  of  Speeton  clay),  which  must  then  have 
lain  beyond  the  estuary  of  the  river.  They  stretch  thence  east- 
wards through  North- Western  Germany,  and  are  found  at  the 
base  of  the  Cretaceous  system  through  France  into  Switzerland. 
The  Lower  Greensand  which  overlies  the  Wealden  group  in  the 
south  of  England  contains  marine  fossils,  and  points  to  the 
submergence  of  the  delta. 

ALBIAN  (from  the  department  of  the  Aube  in  France).  In 
England  this  stage  nearly  corresponds  to  the  band  of  dark,  stiff, 
blue  clay  known  as  the  Gault.  Extending  over  the  Wealden 
sands  and  clays,  the  Gault  (100  to  200  feet  or  more  in  thick- 
ness), with  its  abundant  marine  fossils,  shows  how  thoroughly 
the  Wealden  delta  was  now  submerged  beneath  the  sea. 

CENOMANIAN  (from  Coenomanum,  the  old  Latin  name  of  the 


THE  CRETACEOUS  PERIOD  335 

town  of  Mans  in  the  department  of  Sarthe,  France) .  This  stage 
comprises  a  group  of  impure  chalky,  glauconitic,  and  sandy 
deposits  lying  at  -the  base  of  the  Chalk  in  England  and  the  north 
of  France.  The  subdivisions  of  this  stage  in  England  are  shown 
in  the  following  table  in  descending  order : — 

Gray  chalk  forming  the  base  of  the  Chalk. 
Chalk  Marl   (Red  Chalk  of  Hunstanton). 
Glauconitic  Marl. 
Upper  Greensand. 

Certain  sandy  portions  of  this  group  have  been  called  the  Upper 
Greensand.  The  Glauconitic  (or  Chloritic)  Marl  is  an  impure, 
dull  white,  or  yellowish  chalk,  with  grains  of  glauconite  and 
phosphatic  nodules.  The  Chalk-Marl  is  an  impure  band  of 
chalk  sometimes  overlain  by  a  zone  of  Gray  Chalk  which  forms 
the  base  of  the  true  Chalk-without-flints.  All  these  deposits 
indicate  the  accumulations  of  a  shallow  sea,  probably  not  far 
from  land.  Traced  eastwards  into  Germany,  they  undergo  great 
changes  in  their  lithological  characters,  passing  at  last  in  Saxony 
and  Bohemia  into  sandstones  and  clays  full  of  remains  of  terres- 
trial vegetation,  and  even  including  some  thin  seams  of  coal.  It 
is  in  these  beds  that  the  oldest  dicotyledonous  plants  in  Europe 
have  been  found.  It  is  evident  that  land  existed  in  the  heart  of 
Germany  during  this  stage  of  the  Cretaceous  period.  In  South- 
ern France,  on  the  other  hand,  the  corresponding  strata  are 
massive  hippurite-limestones  which  sweep  through  the  great 
Mediterranean  basin,  and  show  how  large  an  area  of  Southern 
Europe  then  lay  under  the  sea. 

TURONIAN  (from  Touraine).  This  stage  includes  the  lower 
part  of  the  Chalk,  above  the  Gray  Chalk.  The  thick  mass  of 
white  crumbly  limestone  known  as  the  Chalk  has  been  referred 
to  as  the  most  conspicuous  member  of  the  Cretaceous  system  in 
the  west  of  Europe.  It  has  long  been  grouped  into  two  parts, 
a  lower  band  of  "  Chalk-without-flints,"  and  an  upper  band  of 
"  Chalk-with-flints."  The  former  corresponds,  on  the  whole, 
with  the  Turonian  stage.  The  Chalk  is  a  remarkably  pure  lime- 
stone, composed  chiefly  of  crumbled  foraminifera,  urchins,  mol- 
luscs, and  other  marine  organisms.  It  must  have  been  laid  down 
in  a  sea  singularly  free  from  fine  sediment;  but  there  is  no  evi- 
dence that  this  sea  was  one  of  great  depth.  On  the  contrary, 


330  GEOLOGY 

though  the  Chalk  itself  resembles  the  Globigerina  ooze  of  the 
deeper  parts  of  the  Atlantic  Ocean,  the  characters  of  its  forami- 
nifera  and  other  organic  remains  indicate  comparatively  shal- 
low-water conditions.  The  basin  in  which  it  was  laid  down 
shallowed  eastwards,  where,  from  the  evidence  of  sandstones, 
coal-seams,  and  plants,  there  was  land  at  the  time;  while,  prob- 
ably, towards  the  west  there  was  connection  with  the  open  sea. 
The  total  thickness  of  the  Chalk,  including  the  Cenomanian, 
Turonian,  and  Senonian  stages,  exceeds  1,200  feet. 

SENONIAN  (from  Sens,  in  the  department  of  Yonne).  This 
stage  corresponds  generally  with  the  original  English  subdivision 
of  Upper  Chalk,  or  Chalk-with-flints.  Its  most  conspicuous 
feature  is  the  presence  of  the  layers  of  nodules  or  irregular  lumps 
of  black  flint  which  mark  the  stratification  of  the  Chalk.  The 
origin  of  these  concretions  has  been  the  subject  of  much  discus- 
sion among  geologists,  and  it  cannot  be  said  to  have  been  even 
yet  satisfactorily  solved.  Some  marine  plants  (diatoms)  and 
animals  (radiolarians,  sponges,  etc.)  secrete  silica  from  sea- 
water,  and  build  it  up  into  their  framework.  But  the  flints  are 
not  mere  siliceous  organisms,  though  organic  remains  may  often 
be  observed  enclosed  within  them.  They  are  amorphous  lumps 
of  dark  silica,  containing  a  little  organic  matter.  By  some 
process,  not  yet  well  understood,  these  aggregations  of  silica  have 
gathered  usually  round  organic  nuclei,  such  as  sponges,  urchins, 
shells,  etc.  The  decomposition  of  organic  matter  on  the  sea-floor 
may  have  been  the  principal  cause  in  determining  the  abstrac- 
tion and  deposition  of  silica.  Not  infrequently  an  organism, 
such  as  a  brachiopod  or  echinus,  originally  composed  of  car- 
bonate of  lime,  has  been  completely  transformed  into  flint. 

The  Chalk  is  well  exposed  along  the  sea-cliffs  of  the  east  and 
south  of  England.  It  forms  the  promontories  of  Flamborough 
Head,  Dover,  Beachy  Head,  and  the  Needles  in  the  Isle  of 
Wight.  The  white  cliffs  of  Kent  are  repeated  on  the  opposite 
coast  of  France,  where  the  Chalk  with  all  its  lithological  and 
palseontological  characters  reappears,  and  whence  it  extends 
through  Northern  France  into  Belgium. 

DANIAN  (from  Denmark).  This  stage  has  not  been  recog- 
nised in  England.  Its  component  chalky  strata  occur  in  scat- 
tered patches  over  Northern  France,  Belgium,  and  Denmark,  to 
the  south  of  Sweden. 


THE  CRETACEOUS  PERIOD  337 

The  Cretaceous  hippurite-limestones  of  Southern  Europe  and 
the  basin  of  the  Mediterranean  are  prolonged  through  Asia 
Minor  into  Persia,  where  they  cover  a  vast  area.  They  have 
been  found  likewise  on  the  flanks  of  the  Himalaya  Mountains, 
so  that  the  open  Cretaceous  sea  must  have  stretched  right  across 
the  heart  of  the  Old  World.  In  the  Indian  Deccan  a  great  ex- 
tent of  country,  estimated  at  200,000  square  miles,  lies  buried 
under  horizontal  or  nearly  horizontal  sheets  of  lava,  which  have 
a  united  thickness  of  from  4,000  to  5,000  feet,  and  were  erupted 
during  the  Cretaceous  period.  These  eruptions,  from  the  pres- 
ence of  interstratified  layers  containing  remains  of  fresh-water 
shells,  land-plants,  and  insects,  are  believed  to  have  taken  place 
on  land  and  not  under  the  sea. 

Cretaceous  rocks  cover  an  enormous  area  in  North  America. 
They  attain  no  great  thickness  in  the  Eastern  States,  but  they 
thicken  southwards,  until  in  Texas  they  present  massive  lime- 
stones indicative  of  deeper  and  clearer  water  than  elsewhere  in 
that  region.  They  attain  gigantic  proportions  in  Colorado, 
Utah,  and  Wyoming,  whence  they  are  prolonged  northwards  into 
the  British  territories,  with  a  maximum  thickness  of  11,000  to 
13,000  feet.  They  have  yielded  a  remarkably  abundant  and 
varied  series  of  organic  remains.  In  their  upper  parts  (Lara- 
mie  group)  they  contain  a  large  assemblage  of  land-plants,  half 
of  which  are  allied  to  still  living  American  trees,  and  in  some 
places  these  plants  are  aggregated  into  valuable  seams  of  coal. 
The  numerous  reptilian  and  bird  remains  found  in  these  strata 
have  been  already  noticed. 

Rocks  assigned  to  the  Cretaceous  system  cover  a  wide  region 
of  Queensland,  and  also  attain  a  considerable  thickness  in  New 
Zealand. 


888  GEOLOGY 


CHAPTER  XXV. 

THE  TERTIARY  PERIODS. 

THE  Cretaceous  system  closes  the  long  succession  of  Secon- 
dary or  Mesozoic  formations.  The  rocks  which  come 
next  in  order  are  classed  as  Tertiary  or  Cainozoic.  When 
these  names  were  originally  chosen,  geologists  in  general  be- 
lieved not  only  that  the  divisions  into  which  they  grouped  the 
stratified  rocks  of  the  earth's  crust  corresponded  on  the  whole 
with  well-defined  periods  of  time,  but  that  the  abrupt  transi- 
tions, so  often  traceable  between  systems  of  rocks,  served  to  mark 
geological  revolutions,  in  which  old  forms  of  life  as  well  as  old 
geographical  conditions  disappeared  and  gave  place  to  new.  One 
of  the  most  notable  of  such  breaks  in  the  record  was  supposed 
to  separate  the  Cretaceous  system  from  all  the  younger  rocks. 
This  opinion  arose  from  the  study  of  the  geology  of  Western 
Europe,  and  more  especially  of  South-Eastern  England  and 
North-Western  France.  The  top  of  the  Chalk,  partly  worn 
down  by  denudation,  was  found  to  be  abruptly  succeeded  by  the 
pebble-beds,  sands,  and  clays  of  the  lower  Tertiary  groups.  No 
species  of  fossils  found  in  the  Chalk  were  known  to  occur  also 
in  the  younger  strata.  It  was  quite  natural,  therefore,  that  the 
hiatus  at  the  top  of  the  Cretaceous  system  should  have  been  re- 
garded as  marking  the  occurrence  of  some  great  geological  catas- 
trophe and  new  creation,  and,  consequently,  as  one  of  the  great 
divisional  lines  of  the  Geological  Eecord. 

More  detailed  investigation,  however,  has  gradually  over- 
thrown this  belief.  In  Northern  France,  Belgium,  and  Den- 
mark various  scattered  deposits  (Danian,  p.  333)  serve  to  bridge 
over  the  gap  that  was  supposed  to  separate  Mesozoic  and  Caino- 
zoic formations.  In  the  Alps  no  satisfactory  line  could  be  found 
to  separate  undoubtedly  Cretaceous  strata  from  others  as  obvi- 
ously Tertiary.  And  in  various  parts  of  the  world,  especially  in 
Western  North  America,  other  testimony  was  gradually  ob- 


THE  TERTIARY  PERIODS  339 

tained  to  show  that  no  general  convulsion  marked  the  end  of 
the  Secondary  and  beginning  of  the  Tertiary  periods,  but  that 
the  changes  on  the  earth's  surface  proceeded  in  the  same  orderly 
connection  and  sequence  as  during  previous  and  subsequent  geo- 
logical ages.  The  break  in  the  continuity  of  the  deposits  in 
Western  Europe  only  means  that  in  that  part  of  the  world  the 
record  of  the  intervening  ages  has  not  been  preserved.  Either 
strata  containing  the  record  were  never  deposited  in  the  region 
in  question,  or,  having  been  deposited,  they  have  subsequently 
been  removed. 

Bearing  in  mind,  then,  that  such  geological  terms  are  only 
used  for  convenience  of  classification  and  description,  and  that 
what  is  termed  Mesozoic  time  glided  insensibly  into  what  is 
called  Cainozoic,  we  have  now  to  enter  upon  the  consideration  of 
that  section  of  the  earth's  history  comprised  within  the  Tertiary 
or  Cainozoic  periods.  The  importance  of  this  part  of  the  geo- 
logical chronicle  may  be  inferred  from  the  following  facts. 
During  Tertiary  time  the  sea-bed  was  ridged  up  into  land  to 
such  an  extent  as  to  give  the  continents  nearly  their  existing  area 
and  contour.  The  crust  of  the  earth  was  upturned  into  great 
mountain  ranges,  and  notably  into  that  long  band  of  lofty  ground 
stretching  from  the  Pyrenees  right  through  the  heart  of  Europe 
and  Asia  to  Japan.  Some  portions  of  the  Tertiary  sea-bed  now 
form  mountain  peaks  16,000  feet  or  more  above  the  sea.  The 
generally  warm  climate  of  the  globe,  indicated  by  the  world-wide 
diffusion  of  the  same  species  of  shells  in  Palaeozoic  and  less 
conspicuously  in  Mesozoic  time,  now  slowly  passed  into  the 
modern  phase  of  graduated  temperatures,  from  great  heat  at  the 
equator  to  extreme  cold  around  the  poles.  At  the  beginning  of 
the  Tertiary  or  Cainozoic  periods  the  climate  was  mild  even  far 
within  the  Arctic  Circle,  but  at  their  close  it  became  so  cold 
that  snow  and  ice  spread  far  southward  over  Europe  and  North 
America. 

The  plants  and  animals  of  Tertiary  time  are  strikingly  modern 
in  their  general  aspect.  The  vegetation  consists,  for  the  most 
part,  of  genera  that  are  still  familiar  in  the  meadows,  woodlands, 
and  forests  of  the  present  day.  The  assemblage  of  animals,  too, 
becomes  increasingly  like  that  of  our  own  time  as  we  follow  the 
upward  succession  of  strata  in  which  the  remains  are  preserved. 
In  one  strongly  marked  feature,  however,  does  the  Tertiary  fauna 


340  GEOLOGY 

stand  contrasted  alike  with  everything  that  preceded  and  fol- 
lowed it.  If  the  Secondary  periods  could  appropriately  be 
grouped  together  under  the  name  of  the  "  Age  of  Keptiles,"  Ter- 
tiary time  may  not  less  fitly  be  called  the  "  Age  of  Mammals." 
As  the  manifold  reptilian  types  died  out,  the  mammals,  in  ever- 
increasing  complexity  of  organization,  took  their  place  in  the  ani- 
mal world.  By  the  end  of  the  Tertiary  periods  they  had  reached 
a  variety  of  type  and  a  magnitude  of  size  altogether  astonishing, 
and  far  surpassing  what  they  now  present.  The  great  variety 
of  pachyderms  is  an  especially  marked  feature  among  them. 

The  rocks  embraced  under  the  terms  Cainozoic  or  Tertiary 
have  been  classified  according  to  a  principle  different  from  any 
followed  with  regard  to  the  older  formations.  When  they  began 
to  be  sedulously  studied,  it  was  found  that  the  percentage  of 
recent  species  of  shells  became  more  numerous  as  the  strata  were 
followed  from  older  to  newer  platforms.  The  French  naturalist 
Deshayes  determined  the  proportions  of  these  species  in  the  dif- 
ferent Tertiary  groups  of  strata,  and  the  English  geologist  Lyell 
proposed  a  scheme  of  classification  based  on  these  ratios.  His 
names,  with  modifications  as  to  their  application,  have  been  gen- 
erally adopted.  They  are  compounds  of  the  Greek  xawos,  recent, 
with  affixes  denoting  the  proportion  of  living  species. 

To  the  oldest  Tertiary  deposits,  containing  only  about  3  per 
cent  of  living  species  of  shells,  the  name  Eocene  (dawn  of  the 
recent)  was  given.  The  next  series,  containing  a  larger  number 
of  living  species,  has  received  the  name  of  Oligocene  (few 
recent).  The  third  division  in  order  is  named  Miocene,  to 
indicate  that  the  living  species,  though  in  still  larger  proportions, 
are  yet  a  minority  of  the  whole  shells.  The  overlying  series 
forming  the  uppermost  of  the  Tertiary  divisions  is  termed  Plio- 
cene (more  recent),  because  the  majority  are  now  living  species. 
The  same  system  of  nomenclature  has  been  retained  for  the  next 
overlying  group,  which  forms  the  lowest  member  of  the  Post- 
tertiary  or  Quaternary  series.  This  group  is  called  Pleistocene 
(most  recent),  and  all  the  species  of  shells  in  it  are  still  living  at 
the  present  time.  It  must  not  be  supposed  that  the  mere  per- 
centage of  living  or  of  extinct  species  of  shells  in  a  deposit  always 
affords  satisfactory  evidence  of  geological  age.  Obviously,  there 
may  have  been  circumstances  favourable  or  unfavourable  to  the 
existence  of  some  shells  on  the  sea-bottom  which  that  deposit 


THE  TERTIARY  PERIODS  341 

represents,  or  to  the  subsequent  preservation  of  their  remains. 
The  system  of  classification  by  means  of  shell-percentages  must 
be  used  with  some  latitude,  and  with  due  regard  to  other  evidence 
of  geological  age. 

EOCENE. 

In  Europe  great  geographical  changes  took  place  at  the  close 
of  the  Cretaceous  period.  The  wide  depression  in  which  the 
Chalk  had  been  deposited  was  gradually  and  irregularly  elevated, 
and  over  its  site  a  series  of  somewhat  local  deposits  of  clay,  sand, 
marl,  and  limestone  was  laid  down,  partly  in  small  basins  of  the 
sea-floor,  and  partly  in  estuaries,  rivers,  or  lakes.  In  Southern 
Europe,  however,  the  more  open  sea  maintained  its  place,  and 
over  its  floor  were  accumulated  widespread  and  thick  sheets  of 
limestone  which,  from  the  crowded  nummulites  which  they  con- 
tain, are  known  as  Nummulitic  Limestone.  These  characteristic 
rocks  extend  all  over  the  basin  of  the  Mediterranean,  stretching 
far  into  Africa  and  sweeping  eastwards  through  the  Alps,  Car- 
pathians, and  Caucasus,  across  Asia  to  China  and  Japan.  In 
North  America  the  rocks  classed  as  Eocene  present  two  contrast- 
ed types.  Down  the  eastern  and  western  borders  of  the  Con- 
tinent, from  the  coast  of  New  Jersey  into  the  Gulf  of  Mexico 
on  the  one  side,  and  along  the  coast  ranges  of  California  and 
Oregon  on  the  other,  they  are  marine  deposits,  though  occasion- 
ally presenting  layers  of  lignite  with  terrestrial  plants.  Over  the 
vast  plateaux  which  support  the  Eocky  Mountains,  however,  they 
are  of  lacustrine  origin,  and  show  that  in  what  is  now  the  heart 
of  the  Continent  the  bed  of  the  Cretaceous  sea  was  upraised  into 
a  succession  of  vast  lakes,  round  which  grew  a  luxuriant  vegeta- 
tion. In  these  lakes  a  total  mass  of  Eocene  strata,  estimated  at 
not  less  than  12,000  feet,  was  deposited,  entombing  and  preserv- 
ing an  extraordinarily  abundant  and  varied  record  of  the  plant 
and  animal  life  of  the  time. 

The  flora  of  Eocene  time  points  to  a  somewhat  tropical 
climate.  Among  its  plants  are  many  which  have  living  repre- 
sentatives now  in  the  hotter  parts  of  India,  Australia,  Africa,  and 
America.  Above  the  ferns  (Lygodium,  Asplenium,  etc.)  which 
clustered  below,  rose  clumps  of  palms,  cactuses,  and  arc-ids; 
numerous  conifers  and  other  evergreens  gave  the  foliage  an 
umbrageous  aspect,  while  many  deciduous  trees  —  ancestors  of 


342 


GEOLOGY 


some  of  the  familiar  forms  of  our  woodlands  —  raised  their 
branches  to  the  sun.  Among  the  conifers  were  many  cypress- 
like  trees  (Callitris,  Cupressinites) ,  pines  (Pinus,  Sequoia),  and 
yews  (Salisburia  or  Ginko).  Species  of 
aloe  (Agave),  sarsaparilla  (Smilax),  and 
amomum  were  mingled  with  fan-palms 
(Sabal,  Chamcerops)  and  screw-pines 
(Pandanus,  Nipa),  together  with  early 
forms  of  fig  (Ficus),  elm  (Ulmus), 
poplar  (Populus),  willow  (Salix),  hazel 
(Corylus),  hornbeam  (Carpinus),  chest- 
Fig.  193.-Eorne  Plant  nut  (Castanea),  beech  (Fagus),  plane 
(Petrophiloides  Rich-  (Platanus) ,  walnut  (Juglans),  liquid- 
,.  ^onii),  natural  size.  ^^  magnolia?  proteaceJus  plants  (Fig. 

193)  resembling  those  of  Australia  and  the  Cape,  water-bean 
(N elumbium),  water-lily  (Victoria),  maple  (Acer),  gum- 
tree  (Eucalyptus),  cotoneaster,  plum  (Prunus),  almond  (Amyg- 
dalus),  laurel  (Laurus),  cinnamon  tree  (Cinnamomum) 9  and 
many  more. 


Fig.    194. — Eocene    Molluscs,     (a)     Valuta    luctatrix    (g)  ;     (6)     Oliva 
Branderi  (natural  size)  ;    (c)    Cerithium  tricarinatum  (§). 

The  fauna  likewise  points  to  the  extension  of  a  warm  cli- 
mate over  regions  that  are  now  entirely  temperate.  This  is 
particularly  noticeable  with  regard  to  the  mollusca.  The  species 
are,  with  perhaps  a  few  exceptions,  all  extinct,  but  many  of  the 
genera  are  still  living  in  the  warmer  seas  of  the  globe.  Some  of 


THE  TERTIARY  PERIODS  343 

the  most  characteristic  forms  are  species  of  Nautilus,  Oliva,  Va- 
luta, Conus,  Mitra,  Cyrena,  Cytherea,  Chama.  The  genus  of 
Foraminifera,  called  Nummulites  from  the  resemblance  of  the 
organism  to  a  piece  of  money,  is  enormously  abundant  in  the 
limestones  above  referred  to  as  nummulitic  limestones.  It  must 
have  nourished  in  vast  profusion  over  the  floor  of  the  sea,  which 
in  older  Tertiary  time' spread  across  the  heart  of  the  Old  World 
from  the  Atlantic  to  the  Pacific  Oceans.  Some  of  the  most  com- 
mon fish-remains  found  in  the  Eocene  strata  belong  to  the 
genera  Lamna,  Otodus,  MylMates,  Pristis.  Eeptilian  life,  which 
enjoyed  such  a  preponderance  during  the  Mesozoic  ages,  is  con- 
spicuously diminished  in  the  Eocene  deposits  alike  in  number  of 


Fig.  195.  —  Eocene  Mammal   (Palceotherium  magnum, 


individuals  and  variety  of  structure.  'The  genera  are  chiefly  tur- 
tles, tortoises,  crocodiles,  and  sea-snakes,  presenting  in  their 
general  assemblage  a  decidedly  modern  aspect  compared  with  the 
reptilian  fauna  of  the  Secondary  rocks.  Remains  of  birds  aje 
comparatively  rare  as  fossils.  We  have  seen  that  the  earliest 
known  type  has  been  obtained  in  the  Jurassic  system,  and  that 
others  have  been  found  in  the  Cretaceous  rocks.  Still  more  mod- 
ern forms  occur  in  Eocene  strata  ;  they  include  one  (Argillornis) 
which  may  have  been  a  forerunner  of  the  living  gannet  ;  another, 
of  large  size  (Dasornis),  akin  to  the  gigantic  extinct  ostrich-like 
moa  (Dinornis)  of  New  Zealand;  a  third  (Agnopterus)  shows 
an  affinity  with  the  flamingo  ;  while  the  buzzard,  woodcock,  quail, 
pelican,  ibis,  and  African  hornbill  are  represented  by  ancestral 
forms.  That  the  early  type  which  linked  birds  with  reptiles  was 
still  living  is  shown  by  the  remains  of  one  curious  genus  (Odon- 
topteryx)  which  had  serrated  jaws  in  which  the  teeth  were  pro- 
jections of  the  bony  substance. 


GEOLOGY 

^»ut,  as  stated  above,  it  was  chiefly  in  higher  forms  of  life  that 
the  fauna  of  early  Tertiary  time  stood  out  in  strong  contrast 
with  that  of  previous  ages  of  geological  history.  The  mammalia 
now  took- the  leading  place  in  the  animal  world,  which  they  have 
retained  ever  since.  Among  the  Eocene  mammals  reference  may 
here  be  made  to  the  numerous  tapir-like  creatures  which  then 
flourished  (Coryphodon,  PalceotJierium,  Fig.  195,  Anchiloplius, 
etc.).  Some  of  the  forms  were  intermediate  in  character  be- 
tween tapirs  and  horses,  and  included  the  supposed  ancestors 


Fig.  196. —  Skull  of  Tinoceras  ingens  (about  TV). 
of  the  modern  horse  —  small  pony-like  animals,  with  three,  four, 
and  even  traces  of  five  toes  on  each  foot.  Many  of  the  mammals 
of  Eocene  time  presented  more  or  less  close  resemblance  to 
wolves,  foxes,  wolverines,  and  other  modern  forms.  There 
were  likewise  true  opossums.  Numerous  herds  of  hog-like  ani- 
mals (Hyopotamus)  and  of  hornless  deer  and  antelopes  wandered 
over  the  land,  while  in  the  woodlands  lived  early  ancestors  of 
our  present  squirrels,  hedgehogs,  bats,  and  lemurs.  Among 
these  various  tribes  which  recall  existing  genera,  others  of 
strange  and  long-extinct  types  roamed  along  the  borders  of  the 
great  lakes  in  Western  North  America.  The  Tillodonts  were 
a  remarkable  order,  in  which  the  characters  of  the  ungulates, 
rodents,  and  carnivores  were  curiously  combined.  These  ani- 


THE  TERTIARY  PERIODS 


345 


mals,  perhaps  rather  less  in  size  than  the  living  tapir,  had 
skeletons  resembling  those  of  carnivores,  but  with  large  prom- 
inent incisor  teeth  like  those  of  rodents,  and  with  molar  teeth 
possessing  grinding  crowns  like  those  of  ungulates.  Still  more 
extraordinary  were  the  forms  to  which  the  name  of  Deinocerata 
has  been  given  (Deinoceras,  Tinoceras,  Fig.  196).  These  were 
somewhat  like  elephants  in  size,  and  like  rhinoceroses  in  general 
build,  but  the  skull  bore  a  pair  of  horn-like  projections  on  the 
snout,  another  pair  on  the  forehead,  and  one  on  each  cheek. 

The  Eocene  rocks  of  England  are  confined  to  the  south-eastern 
part  of  the  country,  from  the  coast  of  Hampshire  into  Norfolk. 
They  vary  in  character  from  district  to  district,  sands  and 
gravels  being  replaced  by  clays  according  to  the  conditions  in 
which  the  sediment  was  accumulated.  They  are  prolonged  into 
the  north  of  France  and  Belgium.  Arranged  in  tabular  form, 
they  may  be  grouped1  as  follows : — 


ENGLAND. 

FRANCE  AND  BELGIUM. 

I 

ft 

p 

Barton    Clay    and    Upper 
Bagshot  Sands. 

Marine  gypsum  and  marls   of  Paris; 
sands  and  calcareous  sandstones  of 
Belgium   (Wemmelian). 
Sands    (marine),   with   estuarine  and 
fresh-  water  limestones,  etc.    (Sables 
Moyens). 

Middle. 

Bracklesham   Beds    (leaf- 
beds  of  Alum  Bay  and 
Bournemouth),    Middle 
Bagshot  beds. 

Calcaire-grossier     divided     into      (3) 
upper  limestones,   with  marine  and 
fresh-  water     fossils  ;      (2)      middle 
limestones,   with   marine   shells   and 
terrestrial    vegetation;     (1)     lower 
glauconitic   marine    limestones    and 
sands. 
Sandstones  and  sands  (Bruxellian)  of 
Belgium. 

1 

Lower    part    of    Bagshot 
sands. 
London  Clay. 
Oldhaven  Beds. 
Woolwich     and     Reading 
Beds. 
Thanet  Beds. 

Paniselian  sands  of  Belgium. 
Ypresian  clays  and  overlying  sands  of 
Belgium.     Absent  in  Paris  basin. 
Landenian  gravels  and  sands  of  Bel- 
gium. 
Sands    of    Bracheux     (Paris    basin), 
Heersian    beds    of    Belgium,    marls 
of   Meudon;    fresh-water   limestones 
of   Rilly   and    Sezanne.      Limestone 
of  Mons  in  Belgium. 

346  GEOLOGY 

In  striking  contrast  with  these  comparatively  thin  and  locally 
developed  deposits  are  those  of  the  Alps,  Southern  Europe,  and 
the  basin  of  the  Mediterranean.  Masses  of  nummulitic  lime- 
stone and  sandstone,  several  thousand  feet  thick,  have  been 
upraised,  folded,  and  fractured,  and  now  form  important  parts 
of  the  great  mountain  chains  which  run  through  Europe  and 
the  north  of  Africa.  Similar  rocks  have  been  uplifted  along 
the  flanks  of  the  great  chain  of  heights  that  sweeps  through 
the  heart  of  Asia,  reaching  in  the  Himalaya  range  a  height  of 
16,500  feet  above  the  sea-level.  We  thus  learn  not  only  that  a 
large  part  of  the  existing  continents  lay  under  the  sea  during 
Eocene  time,  but  that  the  principal  mountain-chains  of  the 
Old  World  have  been  upheaved  to  their  present  altitudes  since 
the  beginning  of  the  Tertiary  periods.  The  great  Eocene  lake- 
basins  of  North  America  —  so  remarkable  a  feature  in  the 
geography  —  survived  till  a  much  later  part  of  Tertiary  time. 

OLIGOCENE. 

Under  this  name  geologists  have  placed  a  group  of  strata 
usually  of  comparatively  insignificant  thickness,  chiefly  of  fresh- 
water and  estuarine,  but  partly  also  of  marine  origin,  which, 
in  Western  and  Central  Europe,  show  how  the  bays  and  shallow 
seas  of  that  region  in  the  Eocene  period  were  gradually  obliter- 
ated and  replaced  by  land  and  by  sheets  of  fresh  water.  They 
attain  in  Switzerland  a  thickness  of  several  thousand  feet  of 
sandstones,  conglomerates,  and  marls,  almost  entirely  of  lacus- 
trine origin,  and  forming  a  group  of  massive  mountains  (Rigi, 
Rossberg) .  A  large  lake  occupied  -their  site  and  continued  to 
be  an  important  feature  in  the  geography  of  Central  Europe 
during  this  and  the  following  geological  period.  Other  sheets 
of  fresh  water  were  scattered  over  the  west  of  Europe.  One  of 
the  largest  of  these  lay  in  Central  France,  over  the  old  district 
known  as  the  Limagne  d'Auvergne.  In  Germany,  lacustrine 
and  terrestrial  deposits,  including  numerous  seams  of  lignite 
or  brown  coal,  are  separated  by  a  group  of  strata  full  of  marine 
shells,  foraminifera,  etc.,  showing  how  the  lakes  and  wood- 
lands were  submerged  beneath  the  sea.  In  the  Paris  basin, 
and  in  the  Isle  of  Wight,  the  strata  are  chiefly  of  fresh-water 
origin,  but  contain  occasional  marine  intercalations.  Evidently 


THE  TERTIARY  PERIODS  347 

the  Oligocene  period,  throughout  the  European  area,  was  one 
of  considerable  oscillation  in  the  earth's  crust.  During  this 
time,  too,  the  volcanic  eruptions  took  place  whereby  the  great 
sheets  of  basalt  forming  the  terraced  hills  of  the  north  of  Ire- 
land and  Western  Islands  of  Scotland  were  thrown  out. 

An  epoch  of  frequent  change  in  the  relative  positions  of  sea 
and  land  is  one  in  which  there  may  be  exceptional  facilities  for 
the  preservation  of  a  record  of  the  plants  and  animals  of  the 
time.  Oligocene  strata  have  accordingly  a  peculiar  interest 
from  the  abundant  remains  they  contain  of  the  contemporaneous 
terrestrial  plants  and  animals.  The  land  flora  of  that  period 
is  probably  better  known  than  that  of  any  other  section  of  the 
Geological  Record,  chiefly  from  the  extraordinary  abundance 
of  its  remains  which  have  been  preserved  in  the  sediments  of 


Fig.  197. —  Oligocene  Molluscs,     (a)  Ostrea  ventilabrum  (I);   (6)   Cor- 
bula   subpisum    (f)  '»    (c)    Paludina  lenta    (natural   size). 

the  ancient  Swiss  lake.  Judging  of  it  from  these  remains, 
we  learn  that  it  was  in  great  measure  made  up  of  evergreens, 
and  in  various  ways  resembled  the  existing  vegetation  of  tropical 
India  and  Aiistralia  and  that  of  sub-tropical  America,  Its 
fan-palms,  feather-palms,  conifers,  evergreen  oaks,  laurels,  and 
other  evergreen  trees,  gave  a  peculiarly  verdant  umbrageous 
character  to  the  landscape  in  all  seasons  of  the  year,  while 
numerous  proteaceous  shrubs  glowed  with  their  bright  blooms 
on  the  lower  grounds. 

Of  the  terrestrial  fauna  numerous  remains  have  been  found 
in  the  lacustrine  deposits  of  the  time.  We  know  that  the  borders 
of  the  lakes  in  Central  France  were  frequented  by  many  dif- 
ferent kinds  of  birds  —  paroquets,  trogons,  flamingoes,  ibises, 
pelicans,  maraboots,  cranes,  secretary  birds,  eagles,  grouse,  and 
other  forms.  This  association  of  birds  recalls  that  around  the 
lakes  of  Southern  Africa  at  the  present  time.  The  mammals 


348  GEOLOGY 

appeared  in  still  more  numerous  and  abundant  types.  Pachy- 
derms abounded,  including  the  Anoplotherium  —  a  slender, 
long-tailed  animal,  about  the  size  of  an  ass,  with  two  toes  on 
each  foot;  certain  transitional  types  of  ungulates,  with  affinities 
to  the  pigs,  peccaries,  and  chevrotains  (Anthracotherium,  Cheer  o- 
potamus,  Hyopotamus,  etc.) ;  various  forms  of  the  tapir  family, 
and  of  dogs,  civets,  martens,  marmots,  bats,  moles,  and  shrews. 
The  carnivora  still  presented  marsupial  characters,  and  in  not 
a  few  of  the  animal  types  features  of  structure  were  combined 
which  are  now  only  found  in  distinct  genera.  The  Eocene 
pala3otheres  and  the  Oligocene  anoplotheres  appear  to  have  died 
out  before  the  end  of  the  Oligocene  period.  The  fresh  water 
teemed  with  molluscs,  belonging  chiefly  to  genera  that  still 
live  in  our  rivers  and  lakes,  such  as  Unio,  Cyrena,  Paludina, 
Planorbis,  Limncea,  Helix,  and  others  (Fig.  197). 

In  the  Isle  of  Wight  the  highest  Eocene  strata  were  followed 
by  a  group  of  fresh-water,  estuarine,  and  marine  deposits,  for- 
merly classed  as  Upper  Eocene,  but  placed  here  in  the  Oligo- 
cene division.  They  are  arranged  in  the  following  manner  in 
descending  order : — 

Hamstead  series  —  clays,  marls,  and  shelly  layers,  with  fresh-water  and 
estuarine  shells  and  land  plants.  Atyout  260  feet. 

Bembridge  series  —  marls  and  limestone,  with  fresh-water  shells  be- 
low, and  estuarine  shells  above.  About  110  feet. 

Osborne  series  —  clays,  marls,  sands,  and  limestones,  with  abundant 
fresh-water  shells.  About  100  feet. 

Headon  series  —  consisting  of  an  upper  and  lower  division,  containing 
fresh  and  brac&ish  water  fossils  and  a  middle  group  in  which  marine 
shells  and  corals  occur.  About  150  feet. 

These  Isle  of  Wight  strata,  having  a  total  depth  of  more 
than  600  feet,  were  for  many  years  the  only  known  examples  in 
Britain  referable  to  this  portion  of  the  Geological  Eecord,  and 
they  are  still  the  only  beds  in  this  country  which  in  their 
abundant  molluscs  allow  a  comparison  to  be  made  between  them 
and  corresponding  rocks  on  the  Continent.  But  at  Bovey 
Tracey  in  Devonshire  a  small  lake-basin  has  been  discovered, 
the  deposits  of  which  have  yielded  a  large  number  of  terrestrial 
plants  comparable  with  those  found  in  the  Oligocene  strata 
of  Switzerland  and  Germany.  Between  the  great  sheets  of 
basalt,  also,  that  form  the  plateaux  of  Antrim  and  the  Inner 
Hebrides,  numerous  remains  of  a  similar  vegetation  have  been 


THE  TERTIARY  PERIODS 


349 


discovered.  There  can  be  no  doubt  that  these  volcanic  rocks 
were  poured  out  over  the  surface  of  the  land,  and  that  the 
plants,  whose  remains  have  been  disinterred  from  the  inter- 
calated layers  of  tuff  and  hardened  clay,  grew  upon  that  land. 
The  basalts  and  other  lavas,  even  after  the  great  denudation 
which  they  have  undergone,  are  still  in  some  places  more  than 
3000  feet  thick.  They  were  poured  out  in  wide-spreading  sheets 
that  completely  buried  the  previous  topography  and  extended 
as  vast  lava-plains,  like  those  of  younger  date,  which  form  so 
impressive  a  feature  in  the  scenery  of  Montana,  Idaho,  and 
Oregon,  in  Western  North  America. 

In  the  Paris  basin,  the  Oligocene  strata  follow  immediately 
upon  the  Eocene  group  described  on  p.  345.     They  consist  of 

(1)  a  lower  division  of  gypsum    (65  feet)    and  marls,  with 
terrestrial  shells,  and  remains  of  palaBotheres  and  anoplotheres ; 

(2)  a  middle  band  of  marl,  limestone,  and  sand,  with  lacustrine 
and  estuarine  shells;  and  (3)  an  upper  division,  in  which  the 
most  conspicuous  members  are  the  sands  and  hard  siliceous 
sandstone  of  Pontainebleau. 

In  Northern  Germany  the  subjoined  succession  of  strata  in 
descending  order  has  been  noted. 


Upper 
Middle 


Lower 


Marine   marls,   clays,   and   sands. 

Brown  coal  of  the  Lower  Rhine,  with  abundant  terres- 
trial vegetation  and  some  marine  bands. 

Sands  and  Septaria-clay,  with  abundant  marine  fauna; 
occasionally  a  brown-coal  group  occurs. 

Marine  beds  of  Egeln,  with  marine  shells  and  corals. 

Amber  beds  of  Konigsberg,  containing  4  or  5  feet  of 
glauconitic  sand,  with  abundant  pieces  of  amber, 
which  is  the  fossil  resin  of  different  species  of  conifer- 
ous trees.  A  large  number  of  species  of  insects  has 
been  enclosed  and  preserved  in  the  amber. 

Lower  brown  coal  —  sands,  sandstones,  clays,  and  con- 
glomerates, with  interstratified  seams  of  brown  coal 
and  an  abundant  terrestrial  flora,  in  which  coniferse 
are  prominent. 


850  GEOLOGY 


CHAPTER  XXVI. 

TERTIARY  PERIODS  CONTINUED. 

THE  geological  period  at  which  we  are  now  arrived,  one 
of  the  most  important  in  the  history  of  the  configura- 
tion of  the  existing  continents,  embraced  that  portion 
of  geological  time  during  which  the  great  mountain-chains  of 
the  globe  were  uplifted  into  their  present  commanding  positions. 
There  is  good  reason  to  believe  that  these  lines  of  elevation 
are  of  great  geological  antiquity,  and  that  they  have  again  and 
again  been  pushed  upward  during  great  terrestrial  disturb- 
ances. But  the  intervals  between  these  successive  upthrusts 
were  probably  often  of  immense  duration,  so  that  the  mountains, 
being  exposed  .  to  continuous  and  prolonged  denudation,  were 
worn  down,  sometimes  perhaps  almost  to  the  very  roots.  In 
all  probability  the  nucleus  of  the  line  of  the  Alps,  for  example, 
dates  back  to  a  remote  geological  period.  But  only  in  Tertiary 
time  did  it  attain  its  present  dimensions.  We  have  seen  that, 
during  the  Eocene  period,  the  sea  of  the  nummulitic  limestone 
extended  over  at  bast  a  considerable  part  of  the  Alpine  region, 
and  that,  as  the  limestone  now  forms  crumpled  and  dislocated 
mountainous  masses,  the  great  upheaval  of  the  chain  must 
have  taken  place  after  Eocene  time.  Not  improbably  the  proc- 
ess was  a  prolonged  one,  advancing  in  successive  uplifts  with 
intervals  of  rest.  The  final  upheaval  that  gave  the  Alps  their 
colossal  bulk  did  not  take  place  until  the  Miocene  period  or 
later,  for  the  Miocene  strata  have  been  involved  in  the  earth- 
movements,  and  have  been  thrust  up,  bent,  and  broken.  Nor 
were  the  terrestrial  convulsions  confined  to  Central  Europe,  all 
over  the  globe  there  seem  to  have  been  extensive  disturbances. 
The  Eocene  sea-bed  with  its  thick  accumulations  of  nummulite- 
limestone  was  ridged  up  into  land,  and  portions  of  it  were 
carried  upward  on  the  flanks  of  the  mountains,  in  the 
layas  to  a  height  of  16,500  feet  above  the  sea. 


TERTIARY  PERIODS  CONTINUED 


351 


While  these  revolutions  were  taking  place  in  its  topography, 
Europe  continued  to  enjoy  a  climate  which,  to  judge  from  the 
remains  of  plants  and  animals  preserved  in  the  Miocene  rocks, 
must  still  have  been  of  a  somewhat  tropical  character.  The 
flora  that  clothed  the  slopes  of  the  Alps  was  not  unlike  that  of 
the  forests  of  India  and  Australia  at  the  present  time.  Palms 
of  various  kinds  still  flourished  all  over  Central  and  Western 
Europe,  mingled  with  conifers,  laurels,  e^rgreen  oaks,  mag- 
nolias, myrtles,  mimosas,  acacias,  sumac!  s,  figs,  oaks,  and 
various  still  living  genera  of  proteaceous  /  shrubs  (Fig.  198). 
But  there  is  evidence  of  the  incoming  o/.  a  more  temperate 


Fig.  198. —  Miocene  Plants;  (a)  Magnolia  Inglefieldi  (J)  ;  (6)  Rhua 
Meriani  (natural  size)  ;  (c)  Ficus  decandolleana  (i)  ;  (d)  Quercus 
ilicoides  (§). 

climate,  for,  in  the  higher  parts  of  the  Miocene  series  of  strata, 
the  vegetation  was  characterised  by  the  abundance  of  its  beeches, 
poplars,  hornbeams,  elms,  laurels,  pondweeds,  etc. 

Eemains  of  the  terrestrial  fauna  have  been  well  preserved 
in  the  deposits  that  gathered  over  the  floors  of  the  lakes.  We 
know,  for  instance,  that  in  the  woodlands  surrounding  the 
large  Miocene  lake  of  Switzerland  insect  life  was  remarkably 
abundant.  From  the  proportions  of  the  different  kinds  that 
have  been  exhumed,  it  has  been  inferred  that  the  total  insect 
population  was  then  more  varied  in  some  respects  than  it  is 


352  GEOLOGY 

now  in  any  part  of  Europe,  wood-beetles  being  especially 
numerous  and  large.  In  the  thick  underwood,  frogs,  toads, 
lizards,  and  snakes  found  their  food.  Through  the  forests 
there  roamed  antelopes,  deer,  and  three-toed  horses,  while  opos- 
sums, apes,  and  monkeys  (PliopitTiecus,  Dryopitliecus,  Oreo- 
pithecus)  gamboled  among  the  branches.  Wild  cats,  bears 
(Hycenarctos) ,  and  sabre-toothed  lions  (Machairodus)  were 
among  the  prominent  carnivores  of  the  time.  But  the  most 
striking  denizens  of  these  scenes  were  undoubtedly  the  huge 
proboscidian  creatures  among  which  the  Mastodon  and-  Deino- 
therium  took  the  lead.  The  mastodon  (Fig.  199)  was  a  large 
and  long-extinct  form  of  elephant,  which,  besides  tusks  in 
the  upper  jaw,  had  often  also  a  pair  in  the  lower  jaw.  The 


Fig.  199. —  Mastodon  augustidens 

deinotherium  (Fig.  200)  possessed  two  large  tusks  in  the  lower 
jaw  which  were  curved  downwards.  This  huge  animal  prob- 
ably frequented  the  rivers  of  the  time,  using  its  powerful  curved 
tusks  to  dig  up  roots,  and  perhaps  to  moor  itself  to  the  banks. 
Contemporaneous  with  these  colossal  pachyderms  were  species 
of  rhinoceros,  hippopotamus,  and  tapir.  The  rivers  were 
haunted  by  crocodiles,  turtles,  beavers,  and  otters;  while  the 
seas  were  tenanted  by  ancestors  of  our  living  morse,  sea-calf, 
dolphin,  and  lamantin.  It  is  strange  to  reflect  that  such  an 
assemblage  of  animals  should  once  have  found  a  home  all  over 
Europe. 

The   deposits   referable  to   the   Miocene  period   in   Europe 
indicate  a  great  change  in  the  geography  of  the  region  since 


TERTIARY  PERIODS  CONTINUED  353 

Eocene  and  Oligocene  times.  While  most  of  the  Continent 
remained  land,  with  large  lakes  scattered  over  its  surface, 
certain  tracts  had  subsided  beneath  shallow  seas  which  pene- 
trated here  and  there  by  long  arms  into  the  very  heart  of 
the  region.  Britain  continued  to  be  a  land  surface,  and  as 
such  was  continuously  exposed  to  denudation.  Instead  of  the 
formation  of  new  deposits,  there  was  an  uninterrupted  waste 
of  those  already  existing.  So  vast  indeed  has  been  the  destruc- 
tion of  the  Tertiary  strata  of  Britain  that  it  has  evidently 
been  in  progress  for  an  enormous  period  of  time.  Much  of 
it,  no  doubt,  took  place  during  the  long  interval  required  else- 
where for  the  accumulation  of  the  Miocene  series  of  rocks. 


Fig.  200. —  Skull  of  Deinotherium  giganteum  (reduced). 

Not  only  were  the  soft  sands  and  clays  of  the  older  Tertiary 
groups  of  south-eastern  England  worn  away  from  hundreds  of 
square  miles  which  they  originally  covered,  but  even  the  hard 
basalt-sheets  of  Antrim  and  the  Inner  Hebrides  were  so  cut 
down  by  the  various  agents  of  denudation  that  wide  and  deep 
valleys  were  carved  out  of  them,  and  hundreds  of  feet  of  solid 
rock  were  gradually  removed  from  their  surface. 

While  Britain  remained  land,  arms  of  the  sea  spread  over 
what  is  now  Belgium,  and  the  basins  of  the  Loire,  Indre,  and 
Cher,  stretching  across  Southern  France  to  the  Mediterranean, 
passing  along  the  northern  base  of  the  Alps,  running  into  the 
valley  of  the  Ehine  as  far  north  as  Mainz,  sweeping  eastwards 
round  the  eastern  end  of  the  Alps,  and  expanding  into  the 


354  GEOLOGY 

broad  gulf  of  Vienna  among  the  submerged  heights  of  Austria 
and  Hungary. 

The  strata  that  tell  this  story  of  submergence  contain  an 
abundant  assemblage  of  marine  shells,  many  of  which  belong 
to  genera  that  now  live  in  warmer  seas  than  those  which  at 
present  bathe  the  coasts  of  Europe.  Among  them  are  Can- 
cellaria,  Cyprcea,  Mitra,  Murex,  Strombus,  Area,  Cardita, 
Cytherea,  Pectunculus,  Spondylus,  together  with  genera,  such 
as  Ostrea,  Pecien,  Cardium,  Tapes,  Tellina,  which  are  familiar 
in  the  Northern  seas. 

The  district  of  France,  formerly  called  Touraine,  is  largely 
overspread  with  shelly  sands  and  marls,  rarely  more  than  50 
feet  thick,  and  locally  known  as  "  Faluns."  These  deposits 
represent  the  floor  of  the  shallow  Miocene  strait  which  extended 
across  France.  They  have  yielded  upwards  of  300  species  of 
shells,  the  general  character  of  which  marks  a  warmer  climate 
than  now  exists  in  Southern  Europe.  The  tableland  of  Spain, 
with  its  northern  mountainous  border,  rose  along  the  southern 
margin  of  this  strait  which  connected  the  Atlantic  and  the 
Mediterranean.  Through  this  broad  passage  the  large  cetaceans 
of  the  time  passed  freely  from  sea  to  sea,  for  their  bones  are 
found  in  the  upraised  sea-bottom.  The  carcases  of  the  mammals 
that  then  lived  among  the  Pyrenees  —  mastodons,  rhinoceroses, 
lions,  giraffes,  deer,  apes,  and  monkeys  —  were  likewise  swept 
down  into  the  sea.  The  deposits  of  the  shallow  Miocene  straits 
and  bays  thus  supply,  us  with  evidence  of  the  position  of  the 
land  and  the  character  of  its  inhabitants.  Eastwards  the  sea 
appears  to  have  deepened  over  the  region  now  occupied  by  the 
gulf  of  Genoa  and  the  encircling  mountain  ranges,  for  the 
Miocene  deposits  of  that  part  of  the  basin  of  the.  Mediter- 
ranean, consisting  almost  wholly  of  blue  marls,  are  said  to 
reach  the  great  thickness  of  more  than  10,000  feet.  Beyond 
that  depression  the  sea  once  more  shallowed  across  the  site  of 
South-Eastern  Europe.  In  the  Vienna  basin  its  deposits  are 
well  developed  and  consist  of  two  divisions :  ( 1 )  a  lower  group 
(Mediterranean  or  marine  stage)  of  limestones,  marls,  clays,  and 
sands,  containing  an  abundant  assemblage  of  shells,  some  of  which 
belong  to  species  still  living  in  the  present  Mediterranean  Sea, 
or  off  the  west  coast  of  Africa,  and  also  numerous  remains  of 
land-plants  which  again  recall  the  living  floras  of  India  and 


TERTIARY  PERIODS  CONTINUED  355 

Australia;  and  (2)  an  upper  group  (Sarmatian  or  Cerithium 
stage)  of  sands,  gravels,  and  clays  in  which  the  shells  and 
terrestrial  plants  point  to  a  much  more  temperate  climate  than 
that  indicated  by  the  lower  beds. 

On  the  northern  side  of  the  Swiss  Alps,  the  lake  which  was 
formed  by  the  uplifting  of  the  Eocene  sea-floor,  and  in  which 
so  thick  a  succession  of  Oligocene  strata  was  laid  down,  event- 
ually disappeared  among  the  terrestrial  movements  that  sub- 
merged so  much  of  Europe  beneath  the  Miocene  sea.  Marine 
bands  containing  undoubted  Miocene  shells  extend  across 
Switzerland :  but  among  them  there  are  such  abundant  remains 
of  terrestrial  vegetation  as  to  show  that  the  land  was  not  far 
off.  No  doubt  the  Alps,  not  yet  uplifted  to  their  ultimate 
height,  rose  along  the  southern  borders  of  the  strait  that  ran 
across  Central  Europe,  and  bore  on  their  slopes  luxuriant  forest- 
growths.  In  Switzerland,  however,  we  learn  that  before  the 
close  of  the  Miocene  period  the  sea  was  once  more  excluded 
from  the  district,  and  another  lake  made  its  appearance.  The 
marls,  limestones,  and  sandstones  accumulated  in  this  lake 
(CEningen  Beds)  are  among  the  most  interesting  geological 
deposits  in  Europe,  from  the  great  number  and  perfect  preser- 
vation of  the  plants,  insects,  fishes,  and  mammals  which  have 
been  obtained  from  them.  A  large  part  of  our  knowledge 
regarding  the  terrestrial  vegetation  and  animal  life  of  the 
Miocene  period  has  been  derived  from  these  strata. 

Passing  beyond  the  European  area,  we  find  that  some  of  the 
characteristic  vegetation  of  Miocene  time  spread  northwards  far 
within  the  Arctic  Circle.  In  Spitzbergen  and  in  North  Green- 
land, an  abundant  series  of  plant-remains  has  been  discovered, 
including  a  good  many  which  occur  also  as  fossils  in  the  Miocene 
deposits  of  Central  Europe.  More  than  half  of  them  are  trees, 
among  which  are  thirty  species  of  conifers,  also  beeches,  oaks, 
planes,  poplars,  maples,  walnuts,  limes,  and  magnolias.  This 
flora  has  been  traced  as  far  as  81°  45'  north  latitude,  where  the 
last  expedition  sent  out  from  England  found  a  seam  of  coal 
25  to  30  feet  thick,  covered  with  black  shales  full  of  plant- 
remains. 

Miocene  deposits  occupy  a  considerable  area  in  North  Amer- 
ica. In  the  Eastern  States,  they  are  of  marine  origin  and  follow 
generally  the  tract  of  the  underlying  Eocene  beds.  In  the 


356  GEOLOGY 

Western  States  and  Territories  they  are  lacustrine,  and  show 
that  the  lakes  which  covered  so  wide  an  expanse  in  early  Ter- 
tiary time  still  existed,  but  in  greatly  diminished  proportions. 
They  have  preserved  many  interesting  relics  of  the  terrestrial 
life  of  the  period  —  three-toed  horses,  tapiroid  animals,  hogs 
as  large  as  rhinoceroses,  true  rhinoceroses,  huge  elephant-like 
creatures  allied  to  deinoceras  and  tapir,  stags,  camels,  beavers, 
wolves,  bears,  and  lions.  In  India,  also,  thick  masses  of  sedi- 
mentary rock  occur  containing  remains  of  mastodon,  deino- 
therium,  and  other  Miocene  animals. 

PLIOCENE. 

The  last  division  of  the  Tertiary  series  of  formations  lays 
before  us  the  history  of  the  geological  changes  that  brought 
about  the  present  general  distribution  of  land  'and  sea,  and 
completed  the  existing  framework  of  the  continents.  Con- 
trasted with  the  previous  Tertiary  groups,  it  is,  on  the  whole, 
insignificant  in  thickness  and  extent,  and  it  probably  records 
the  passing  of  a  much  less  period  of  time,  during  which  the 
amount  of  terrestrial  revolution  was  comparatively  trifling. 
Only  in  the  basin  of  the  Mediterranean  are  there  any  European 
Pliocene  strata  worthy  of  note  on  account  of  their  thickness. 
The  floor  of  that  sea  slowly  subsided  until  sands,  clays,  and 
accumulated  shelj-beds  had  been  piled  up  to  a  depth  of  several 
thousand  feet.  An  important  volcanic  episode  then  took  place. 
Etna,  Vesuvius,  and  the  other  volcanoes  of  Central  Italy  began 
their  eruptions.  Thick  masses  of  Pliocene  sediments  were 
ridged  upon  both  sides  of  the  Apennines,  and  in  Sicily  were 
upheaved  to  a  height  of  nearly  4000  feet  above  the  present  sea- 
level.  This  elevation  of  the  Pliocene  sea-bed  in  the  Mediter- 
ranean area  was  not  improbably  connected  with  other 
movements  within  the  European  region.  The  shallow  firths  and 
bays  which  still  indented  the  Continent  were  finally  raised  into 
dry  land,  and  the  Alps  may  then  have  received  their  final  uplift. 
While  the  European  Pliocene  deposits  have  their  maximum 
thickness  in  the  Mediterranean  basin,  they  elsewhere  represent 
the  sediments  of  shallow  seas  and  of  lakes  and  rivers. 

The  flora  of  the  Pliocene  period  affords  evidence  of  the  con- 
tinued advance  of  a  more  temperate  climate.  The  tropical 


TERTIARY  PERIODS  CONTINUED 


357 


types  of  vegetation  one  by  one  retreated  southwards  in  the 
European  region,  leaving  behind  them  a  vegetation  that  par- 
took of  the  characters  of  those  of  the  present  Canary  Islands, 
of  North  America,  and  of  Eastern  Asia  and  Japan,  but  which, 
as  time  wore  on,  approached  more  and  more  to  the  present 


Fig.  201. —  Pliocene  Plants.  (A)  Populus  canescens;  (B)  Salix  alba; 
((7)  Glyptostrobus  europceus;  (D)  Alnus  glutinosa;  (E)  Platanus 
aceroides  (all  natural  size  except  E,  which  is  i). 

European  flora  (Fig.  201).  It  included  species  of  bamboo, 
sarsaparilla  (Smilax),  ^lyptostrobus,  taxodium,  sequoia,  mag- 
nolia, tulip  tree  (Liriodendron) ,  maple  (Acer),  buckthorn 
(Rliamnus),  sumach  (Rhus),  plum  (Prunus),  laurel  (Laurus), 
cinnamon-tree  (Cinnamomum),  sassafras,  fig  (Ficus),  elm 


358 


GEOLOGY 


(Ulmus),  willow  (Salix),  poplar  (Populus),  alder  (Alnus), 
birch  (Betula),  liquidambar,  oak  (Quercus),  evergreen  oak 
(Quercus  ilex),  plane  (Platanus) ,  walnut  (Juglans),  hickory 
(Carya),  and  other  now  familiar  trees. 

The  fauna  presented  likewise  evidence  that  the  climate,  dur- 
ing at  least  the  earlier  part  of  the  Pliocene  period,  still  continued 
warm  enough  to  permit  tribes  of  animals  to  roam  over  Europe, 
the  descendants  of  which  are  now  confined  to  regions  south  of 
the  Mediterranean  basin.  Some  of  the  huge  mammalian  types 
that  had  survived  from  an  earlier  time  now  died  out;  such  was 


Fig.  202.— Pliocene  Marine  Shells,  (a)  Rhynchonella  psittacea  (natural 
size)  ;  (6)  Panopcea  norvegica  (J)  ;  (c)  Purpura  lapillus  (i)  ;  (d) 
Trophon  antiquum  (|). 

the  case  with  the  deinotherium  and  mastodon.  Herds  of  pachy- 
dermatous animals  formed  a  distinguishing  feature  of  the 
fauna  —  rhinoceroses,  hippopotamuses,  and  elephants,  with 
troops  of  herbivorous  quadrupeds  —  gazelles,  antelopes,  deer, 
giraffes,  horses,  oxen,  and  strange  long-extinct  types  linking 
together  genera  that  are  now  quite  distinct.  There  were,  like- 
wise, carnivores  (wild-cats,  bears,  hyenas,  etc.),  and  many 
monkeys.  The  remains  of  monkeys  have  been  found  fossil  in 
Europe  1-1°  farther  north  than  their  descendants  now  live. 


TERTIARY  PERIODS  CONTINUED 


359 


The  shells  of  the  Pliocene  deposits  afford  important  evidence 
regarding  the  gradual  change  of  climate.  The  great  majority 
of  them  belong  to  still  living  species  (Fig.  202).  They  con- 
sequently supply  an  excellent  basis  for  comparison  with  the 
existing  distribution  of  the  same  species.  When  the  deposits 
containing  them  are  examined  with  reference  to  the  present 
habitats  of  the  species,  it  is  found  that  the  percentage  of  what 
are  now  northern  shells  increases  from  the  lower  to  the  higher 
parts  of  the  series.  In  Pliocene  time,  each  species  no  doubt 
flourished  only  in  that  part  of  the  sea  where  it  found  its  con- 
genial temperature  and  food.  We  infer  that  its  requirements 
are  still  the  same  at  the  present  day,  in  other  words,  that  the 
temperature  of  the  regions  within  which  the  species  is  now 
confined  affords,  on  the  whole,  an  indication  of  the  temperature 
of  the  areas  within  which  it  lived  in  the  Pliocene  seas.  On 
this  basis  of  comparison,  the  inference  has  been  drawn  that 
the  climate  in  the  northern  hemisphere,  after  becoming  tem- 
perate, passed  on  to  a  more  rigorous  stage.  In  the  end  thor- 
oughly Arctic  conditions  spread  over  most  of  Europe  and  a 
large  part  of  North  America,  during  the  period  that  succeeded 
the  Pliocene. 

In  Britain  Pliocene  deposits  are  almost  entirely  confined  to 
the  counties  of  Norfolk  and  Suffolk.  They  consist  of  various 
shelly  sands,  gravels,  and  marls,  which  have  long  been  known  as 
"  Crag."  Arranged  in  descending  order,  the  following  are 
the  recognised  subdivisions : — 

'  Upper  fresh-water,  estuarine,  and  Lower  fresh-water 
sands  and  silts,  with  layers  of  peat,  having  a  total 
depth  of  10  to  70  feet.  Among  the  terrestrial  plants 
are  cones  of  Scotch  fir  (Pinus  sylvestris)  and 
spruce  (Abies),  leaves  of  water-lily  (Nymphcea  al- 
60),  yellow  pond-lily  (Nuphar  luteum) ,  hornwort 
(Ceratophyllum) ,  blackthorn  (Prunus  spinosa), 
Forest  -  Bed  bog-bean  (Menyanthes  trifoliata) ,  oak,  and  hazel, 
Group  -j  with  land  and  fresh-water  shells,  and  many  mam- 
mals, including  species  of  wolf,  fox,  machairodus, 
hyaena,  glutton,  bear,  seal,  horse,  rhinoceros,  hip- 
popotamus, pig,  ox,  musk-sheep,  deer,  beaver,  trogon- 
therium  (a  huge  extinct  kind  of  beaver),  mole, 
elephant  (E.  antiquus,  E.  meridionalis,  E.  primi- 
genius) ,  etc.  This  group  of  strata  is  found  at  the 
base  of  the  sea-cliff  of  boulder-clay  in  Norfolk, 
and  extends  under  the  present  sea. 


360 


GEOLOGY 


Chillesford 
Group 


Norwich  (flu- 
vio-marine  or 
mammalife  r  - 
ous)  Crag 


Red  Crag 


Lenham  Beds 
(Diestian) 


St.   Erth  Beds 


White  (Suffolk 
or  Coralline) 
Crag 


Sands  and  clays  occurring  as  a  thin  local  deposit  in 
Suffolk,  6  to  16  feet  thick,  with  marine  shells,  about 
two-thirds  of  which  still  live  in  Arctic  waters  (My a 
truncata,  Cyprina  islandica,  Astarte  borealis,  Tel- 
Una  olliqua). 

Shelly  sand  and  gravel,  5  to  10  feet  thick,  containing 
93  per  cent  of  still  living  species  of  shells  and  bones 
and  teeth  of  mastodon,  elephant  (E.  meridionalis,  E. 
antiquus) ,  hippopotamus,  rhinoceros,  etc.  The  pro- 
portion of  northern  shells  is  14.6  per  cent,  and  the 
following  species  are  included — Rhynchonella  psit- 
tacea,  Scalaria  grcenlandica,  Panopwa  norvegica,  As- 
tarte borealis.  About  twenty  species  of  land  or 
fresh-water  shells  also  occur. 

A  local  and  inconstant  accumulation,  25  feet  thick,  of 
red  and  dark  brown  ferruginous  shelly  sand,  with 
numerous  species  of  shells  of  which  10.7  per  cent 
are  northern  forms.  Some  of  the  characteristic  shells 
of  the  deposit  are  —  Trophon  antiquum,  Valuta 
Lamberti,  Purpura  lapillus,  Pectunculus  glycimeris, 
Cardium  edule. 

Sands  and  ironstones  filling  hollows  of  the  Chalk  of  the 
North  Downs,  more  than  600  feet  above  the  sea,  and 
containing  nearly  200  species  of  fossils,  all  of  which, 
save  22,  have  been  found  in  the  Coralline  Crag. 

A  local  deposit  of  clays  and  gravels  found  at  St.  Erth 
in  Cornwall,  with  abundant  and  well-preserved 
shells,  probably  of  older  Pliocene  age,  about  40  per 
cent  being  of  extinct  species. 

Shelly  sands  and, clays  containing  84  per  cent  of  still 
living  shells,  whereof  5  per  cent  are  northern  species. 
One  of  the  characteristics  of  the  deposit  is  the  large 
number  (140  species)  of  coral-like  polyzoa  (coral- 
lines or  bryozoa),  whence  one  of  the  names  given  to 
this  subdivision. 


On  the  Continent  the  youngest  Tertiary  deposits  cover  com- 
paratively small  areas  and  mark  some  of  the  last  tracts  occupied 
by  the  sea.  Thus,  in  the  Vienna  basin  there  is  evidence  that 
the  sea,  shut  off  from  the  main  ocean,  and  partly  converted 
into  an  inland  sea,  like  the  Caspian,  was  gradually  filled  up 


TERTIARY  PERIODS  CONTINUED  361 

with  sediment  and  raised  into  land.  Along  the  northern  borders 
of  the  Mediterranean  Sea,  thick  masses  of  marine  Pliocene 
strata  show  the  prolonged  depression  of  that  region  during 
Pliocene  time,  and  its  subsequent  elevation.  In  the  south  of 
France  these  strata,  lying  unconfonnably  on  everything  older 
than  themselves,  reach  a  height  of  1150  feet  above  the  sea. 
Along  both  sides  of  the  Apennine  chain,  Pliocene  blue  marls, 
clays,  and  sands,  known  as  the  sub-Apennine  beds,  have  been 
uplifted  into  a  range  of  low  hills.  These  deposits  swell  out 
southwards,  reaching  their  greatest  thickness  (2000  feet  or 
more)  in  Sicily,  which  was  probably  the  region  of  maximum 
subsidence  during  Pliocene  time.  Here  and  there,  in  the 
Italian  strata  of  this  period,  remains  of  terrestrial  vegetation 
and  land-animals  are  abundantly  preserved.  One  of  the  most 


Fig.  203. —  Helladotherium  Duvernoyi   (-fa) — a  gigantic  animal  belong- 
ing to  the  same  family  as  the  living  giraffe,  Pikermi,  Attica. 

noted  localities  for  these  fossils  is  the  upper  part  of  the  valley 
of  the  Arno. 

Perhaps  the  most  curious  and  interesting  assemblage  of  the 
land-fauna  of  Europe  during  Pliocene  time  has  been  found  in 
some  hard  red  clays,  alternating  with  gravels,  at  Pikermi  in 
Attica.  Thirty-one  genera  of  mammals  have  there  been  ob- 
tained, of  which  twenty-two  are  extinct.  The  ruminants,  spe- 
cially well  represented  among  these  remains,  include  species 
of  giraffe,  helladotherium  (Fig.  203),  antelopes,  gazelles,  and 
other  forms  allied  to,  but  distinct  from,  any  living  genera. 
There  are  likewise  the  bones  of  gigantic  wild  boars,  several 


362  GEOLOGY 

species  of  rhinoceros,  mastodon,  deinotherium,  porcupine,  hyaena, 
various  extinct  carnivores,  and  a  monkey. 

In  India  a  somewhat  similar  fauna  has  been  obtained  from 
a  massive  series  of  fresh-water  sandstones,  known  as  the  Siwalik 
group.  A  large  proportion  of  the  remains  belong  to  existing 
genera  of  animals,  such  as  macaque,  bear,  elephant,  horse,  hippo- 
potamus, giraffe,  ox,  porcupine,  goat,  sheep,  and  camel.  Various 
extinct  types  were  contemporary  with  these  animals,  two  of 
the  most  extraordinary  of  them  being  the  Sivatherium  and 
Bramatherium  —  colossal,  four-horned  creatures  allied  to  our 
living  antelopes  and  prong-bucks. 


POST-TERTIARY  PERIODS  363 


CHAPTEE  XXVII. 

POST-TERTIAEY  PEKIODS. 

WE  HAVE  now  arrived  at  the  last  main  division  of  the 
Geological  Record,  that  which  is  named  POST-TER- 
TIARY or  QUATERNARY,  and  which  includes  all  the 
formations  accumulated  from  the  close  of  the  Tertiary  periods 
down  to  the  present  day.  But  no1  sharp  line  can  be  drawn  at 
the  top  of  the  Tertiary  groups  of  strata.  On  the  contrary,  it  is 
often  difficult,  or  indeed  impossible,  satisfactorily  to  decide 
whether  a  particular  deposit  should  be  classed  among  the  younger 
Tertiary  or  among  the  Post-tertiary  groups.  In  the  latter,  all 
the  molluscs  are  believed  to  belong  to  still  living  species,  and 
the  mammals,  although  also  mostly  still  of  existing  species, 
include  some  which  have  become  extinct.  These  extinct  forms 
are  numerous  in  proportion  to  the  antiquity  of  the  deposits  in 
which  they  have  been  preserved.  Accordingly,  a  classification 
of  the  Quaternary  strata  has  been  adopted,  in  which  the  older 
portions,  containing  a  good  many  extinct  mammals,  have  been 
formed  into  what  is  termed  the  Pleistocene,  Post-pliocene,  or 
Glacial  group,  while  the  younger  deposits,  containing  few  or 
no  extinct  mammals,  are  termed  Recent. 

The  gradual  refrigeration  of  climate  which  is  revealed  to 
us  by  the  shells  of  the  crag  was  prolonged  and  intensified 
in  Post-tertiary  time.  Ultimately  the  northern  part  of  the 
northern  hemisphere  was  covered  with  snow  and '  ice,  which 
extended  into  the  heart  of  Europe  and  descended  far  southward 
in  North  America.  The  previous  denizens  of  land  and  sea  were 
in  large  measure  driven  out  or  even  in  many  cases  wholly  extir- 
pated by  the  cold,  while  northern  forms  advanced  southward  to 
take  their  places.  The  reindeer,  for  instance,  roamed  in  great 
numbers  across  Southern  France,  and  Arctic  vegetation  spread 
all  over  Northern  and  Central  Europe,  even  as  far  as  the 
Pyrenees.  After  the  cold  had  reached  its  climax,  the  ice-fields 


364  GEOLOGY 

began  to  retreat,  and  the  northern  flora  and  fauna  to  retire 
before  the  advance  of  the  plants  and  animals  which  had  been 
banished  by  the  increasingly  severe  temperature.  And  at  last 
the  present  conditions  of  climate  were  reached.  The  story  of 
this  Ice  Age  is  told  by  the  Pleistocene  or  Post-pliocene  forma- 
tions, while  that  of  the  changes  which  immediately  led  to  the 
establishment  of  the  present  order  of  things  is  made  known 
in  the  Recent  deposits. 

PLEISTOCENE,  POST-PLIOCENE,  OR  GLACIAL. 

The  evidence  from  which  geologists  have  unravelled  the  his- 
tory of  the  Ice  Age  or  cold  episode  which  came  after  the  Tertiary 
periods  in  the  northern  hemisphere  may  here  be  briefly  given. 
All  over  Northern  Europe  and  the  northern  part  of  North 
America  the  solid  rocks,  where  of  hardness  sufficient  to  retain 
it,  are  found  to  present  a  characteristic  smoothed,  polished,  and 
striated  surface.  Even  on  crags  and  rocky  bosses  that  have 
remained  for  long  periods  exposed  to  the  action  of  the  weather, 
this  peculiar  worn  surface  may  be  traced;  but  where  they  have 
been  protected  by  a  covering  of  clay,  these  markings  are  often 
as  fresh  as  when  they  were  first  made.  The  groovings  and  fine 
striae  do  not  occur  at  random,  but  in  every  district  run  in  one 
or  more  determinate  directions.  The  faces  of  rock  that  look 
one  way  are  rounded  off,  smoothed,  and  polished;  those  that 
face  to  the  opposite  quarter  are  more  or  less  rough  and  angular. 
The  quarter  to  which  the  worn  faces  are  directed  corresponds 
with  that  to  which  the  striae  and  grooves  on  the  rock-surfaces 
point.  There  can  be  no  doubt  that  all  this  smoothing,  polishing, 
grooving,  and  striation  has  been  done  by  land-ice ;  that  the  trend 
of  the  striae  marks  the  direction  in  which  the  ice  moved,  those 
faces  of  rock  which  looked  towards  the  ice  being  ground  away, 
while  those  that  looked  away  from  it  escaped.  By  following 
out  the  directions  of  the  rock-striae  we  can  still  trace  the  march 
of  the  ice  across  the  land  (see  Chapter  VI). 

As  the  ice  travelled,  it  carried  with  it  more  or  less  detritus, 
as  a  glacier  does  at  the  present  day.  Some  of  this  material 
may  have  lain  on  the  surface,  but  probably  most  of  it  was 
pushed  along  at  the  bottom  of  the  ice.  Accordingly,  above 
the  ice-worn  surfaces  of  rock,  there  lies  a  great  deposit  of 


POST-TERTIARY  PERIODS  365 

clay  and  boulders,  evidently  the  debris  that  accumulated  under 
the  ice-sheet  and  was  left  on  the  surface  of  the  ground  when 
the  ice  retired.  This  deposit,  called  Boulder-Clay  or  Till,  bears 
distinct  corroborative  testimony  to  the  movement  of  the  ice. 
It  is  always  more  or  less  local  in  origin,  but  contains  a  variable 
proportion  of  stones  which  have  travelled  for  a  greater  or  less 
distance,  sometimes  for  several  hundred  miles.  When  these 
stones  are  traced  to  their  places  of  origin,  which  are  often  not 
hard  to  seek,  they  are  found  to  have  come  from  the  same  quarter 
as  that  indicated  by  the  striation  of  the  rocks.  If,  for  example, 
the  ice-worn  bosses  of  rock  show  the  ice  to  have  crept  from 
north  to  south,  the  boulders  will  be  found  to  have  a  northern 
origin.  The  height  to  which  striated  rock-surfaces  and  scattered 
erratic  blocks  can  be  traced  affords  some  measure  of  the  depth 
of  the  ice-sheet. 

From  this  kind  of  evidence  it  has  been  ascertained  that  the 
whole  of  Northern  Europe,  amounting  in  all  to  probably  not 
less  than  770,000  square  miles,  was  buried  under  one  vast  ex- 
panse of  snow  and  ice.  The  ice-sheet  was  thickest  in  the  north 
and  west,  whence  it  thinned  away  southward  and  eastward. 
Upon  Scandinavia  it  was  not  improbably  between  6000  and 
7000  feet  thick.  It  has  left  its  mark  at  heights  of  more  than 
3000  feet  in  the  Scottish  Highlands,  and  over  North-Western 
Scotland  it  was  probably  not  less  than  5000  feet  thick.  Where 
it  abutted  upon  the  range  of  the  Harz  Mountains,  it  appears 
to  have  been  still  not  far  short  of  1500  feet  in  thickness. 

This  vast  mantle  of  ice  was  in  continual  motion,  creeping 
outward  and  downward  from  the  high  grounds  to  the  sea.  The 
direction  taken  by  its  principal  currents  can  still  be  followed. 
In  Scandinavia,  as  shown  by  the  rock-stria3  and  the  transport 
of  boulders,  it  swept  westward  into  the  Atlantic,  eastward  into 
the  Gulf  of  Bothnia,  which  it  completely  filled  up,  and  south- 
ward across  Denmark  and  the  low  grounds  of  Northern  Ger- 
many. The  basin  of  the  Baltic  was  completely  choked  up 
with  ice;  so  also  was  that  of  the  North  Sea  as  far  south  as  the 
neighbourhood  of  London.  From  the  same  evidence  we  know 
that  the  ice  which  streamed  off  the  British  Islands  moved 
eastward  from  the  slopes  of  Scotland  into  the  hollow  of  the 
North  Sea,  part  of  it  turning  to  the  left  to  join  the  south-western 
margin  of  the  Scandinavian  sheet^  and  move  with  it  northwards 


366  GEOLOGY 

and  westwards  across  the  Orkney  and  Shetland  Islands  into 
the  Atlantic,  and  another  branch  bending  southwards  and  mov- 
ing with  the  southerly  expansion  of  the  Scandinavian  ice  along 
the  floor  of  the  North  Sea  and  the  low  grounds  of  the  east  of 
England;  and  that  on  the  west  side  of  Scotland  the  ice  filled 
up  and  crept  down  all  the  fjords,  burying  the  Western  Islands 
under  its  mantle  and  marching  out  into  the  Atlantic.  The 
western  margin  of  the  ice-fields,  from  the  south-west  of  Ireland 
to  the  North  Cape  of  Norway,  must  have  presented  a  vast  wall 
of  ice  some  2000  miles  long,  and  probably  several  hundred  feet 
high,  breaking  off  into  icebergs  which  floated  away  with  the 
prevailing  currents  and  winds.  The  Irish  Sea  was  likewise 
filled  with  ice,  moving  in  a  general  southerly  direction. 

Northern  Europe  must  thus  have  presented  the  aspect  of 
North  Greenland  at  the  present  time.  The  evidence  of  rock- 
striae  and  ice-borne  blocks  enable  us  to  determine  approx- 
imately the  southern  limit  to  which  the  great  ice-cap  reached. 
As  even  the  southern  coast  of  Ireland  is  intensely  ice-worn,  the 
edge  of  the  ice  must  have  extended  some  distance  beyond  Cape 
Clear,  rising  out  of  the  sea  with  a  precipitous,  front  that  faced 
to  the  south.  Thence  the  ice-cliff  swung  eastwards,  passing 
probably  along  the  line  of  the  Bristol  Channel  and  keeping  to 
the  north  of  the  valley  of  the  Thames. 

That  the  northern  ice  moved  down  the  bed  of  the  North 
Sea  is  shown  by  the  boulder-clays  and  transported  stones  of  the 
eastern  counties  of  England,  among  which  fragments  of  well- 
known  Norwegian  rocks  are  recognisable.  Its  southern  margin 
ran  across  what  is  now  Holland,  and  skirted  the  high  grounds 
of  Westphalia,  Hanover,  and  the,Harz,  which  probably  there 
arrested  its  southward  extension.  There  is  evidence  that  the 
ice  swept  round  into  the  Lowlands  of  Saxony  up  to  the  chain 
of  the  Erz,  Eiesen,  and  Sudeten  Mountains,  whence  its  southern 
limit  turned  eastward  across  Silesia,  Poland,  and  Galicia,  and 
then  swung  round  to  the  north,  passing  across  Eussia  by  way  of 
Kieff  and  Nijni  Novgorod  to  the  Arctic  Ocean. 

In  Europe  no  distinct  topographical  feature  appears  to  mark 
the  southern  limit  reached  by  the  ice-sheet;  this  limit  can  only 
be  approximately  fixed  by  the  most  southerly  localities, .  where 
striated  rocks  and  transported  blocks  Tiave  been  observed.  In 
North  America,  however,  the  margin  of  the  great  ice-cap  is 


POST-TERTIARY  PERIODS  3G7 

prominently  defined  by  a  mound  or  series  of  mounds  of  detritus 
which  seem  to  have  been  pushed  in  front  of  the  ice.  These 
mounds,,  beginning  on  the  coast  of  Massachusetts,  run  across 
the  Continent  with  a  wonderful  persistence  for  more  than  3000 
miles.  They  form  what  American  geologists  call  the  "  terminal 
moraine/' 

The  detritus  left  by  the  ice-sheet  consists  of  earthy,  sandy, 
or  clayey  material  (Boulder-Clay,  Till)  more  or  less  charged 
with  stones  of  all  sizes  up  to  blocks  weighing  many  tons.  For 
the  most  part  it  is  unstratified,  and  bears  witness  to  the  irregu- 
lar way  in  which  it  was  tumbled  down  by  the  ice.  In  some 
districts,  it  has  been  more  or  less  arranged  in  water,  and  then 
assumes  a  stratified  character.  The  stones  in  the  detritus,  more 
especially  where  they  are  hard  and  are  imbedded  in  a  clayey 
matrix,  present  smooth  striated  surfaces,  the  striae  usually  run- 
ning along  the  length  of  the  stone,  but  not  infrequently  crossing 
each  other,  the  older  being  partially  effaced  by  a  newer  set  (Fig. 
24).  This  characteristic  striation  points  unmistakably  to  the 
slow  creeping  motion  of  land-ice. 

But  the  boulder-clays,  earths,  and  gravels  left  by  the  great 
ice-sheet  are  not  simply  one  continuous  deposit.  On  the  con- 
trary, they  contain  intercalations  of  stratified  sand,  clay,  and 
even  peat.  In  these  included  strata  organic  remains  occur,  for 
the  most  part  those  of  terrestrial  plants  and  animals,  showing 
that  the  ice  again  and  again  retreated,  leaving  the  country  to 
be  covered  with  vegetation,  and  to  be  tenanted  by  land  animals ; 
but  that  after  longer  or  shorter  periods  of  diminution  it  once 
more  advanced  southward  over  its  former  area.  These  intervals 
of  retreat  are  known  as  "  interglacial  periods/'  Probably  they 
were  of  prolonged  duration,  the  climate  becoming  comparatively 
mild  and  equable  while  they  lasted.  The  occurrence  of  boulder- 
clays  above  the  interglacial  deposits  shows  a  subsequent  lowering 
of  the  temperature,  with  a  consequent  renewal  of  glacial  con- 
ditions. 

The  Pleistocene  deposits  thus  reveal  to  us  a  prolonged  period 
of  cold  broken  up  by  shorter  intervals  of  milder  climate.  The 
fossils  which  they  contain  throw  curious  and  interesting  light 
on  these  oscillations  of  temperature.  Among  the  plants,  leaves 
of  Arctic  species  of  birch  and  willow  are  found  far  to  the  south 
of  their  present  limits;  on  the  other  hand,  remains  of  plants 


3G8 


GEOLOGY 


now  confined  to  temperate  latitudes  are  found  fossil  in  Siberia^ 
and  others,  now  living  in  more  genial  climates  than  those  of 
Central  Europe,  are  associated  in  interglacial  deposits,  with  the 
remains  of  the  still  indigenous  vegetation. 

To  the  same  effect,  but  still  more  striking,  is  the  testimony 
of  the  Pleistocene  fauna,  with  its  strange  mingling  of  northern 


Fig.  204. —  Pleistocene  or  Glacial  Shells,  (a)  Pecten  islandicus  (J)  ; 
(&)  Leda  truncata  (£)  ;  (c)  Leda  lanceolata  (!)  ;  (d)  Tellina  lata 
(!)  ;  (e)  Satvicava  rugosa  (j)  ;  (/)  Natica  clausa  (%)  ;  (g)  Trophon 
scalari forme  ( \ ) . 

and  southern  forms.  The  marine  shells  imbedded  in  the  glacial 
clays  of  Scotland,  though  chiefly  belonging  to  species  that  still 
live  in  the  adjoining  seas,  include  a  few  that  are  now  restricted 
to  more  northern  latitudes  (Pecten  islandicus,  Leda  lanceolata, 
Tellina  lata,  etc.,  Fig.  204) .  Turning  to  the  terrestrial  mam- 
mals, we  find  among  the  Pleistocene  deposits  the  remains  of 
the  last  of  the  huge  pachyderms  which,  through  Tertiary  time, 
had  been  so  striking  a  feature  of  the  animal  population  of 
Europe.  The  hairy  mammoth  (ElepJias  primigenius,  Fig.  205) 


POST-TERTIARY  PERIODS 


369 


and  the  woolly  rhinoceros  (R.  tichorhmus)  now  roamed  all  over 
the  Continent  and  across  Britain,  which  had  not  yet  become  an 
island.  During  the  retreat  of  the  snow  and  ice,  they  found 


Fig.  205. —  Mammoth    (Elephas  primigenius)    from   the  skeleton   in   the 
Musee   Royal,   Brussels. 

their  way  into  the  forests  and  pastures  of  Northern  Siberia. 
Driven  southwards  when  the  cold  increased,  they  were  accom- 
panied by  numerous  Arctic  animals  which  have  not  yet  become 
extinct.  Herds  of  reindeer  (Cervus  tarandus)  sought  the  pas- 
tures of  Central  France  and  Switzerland;  the  glutton  (Gulo 
luscus)  came  to  the  South  of  Eng- 
land and  to  Auvergne  ;  the 
musk-sheep  (Ovibos  moschatus, 
Fig.  206)  and  Arctic  fox  (Cam's 
lag  opus)  wandered  southward 
to  the  Pyrenees.  But  as  each 
oscillation  of  climate  slowly 
brought  in  a  milder  temperature, 
and  pushed  the  snow  and  ice 
northward,  animals  of  southern 
types  made  their  way  into 
Southern  and  Central  Europe. 
Among  these  immigrants  were  the  porcupine  (Hystrix) ,  leopard 
(Felis  pardus),  African  lynx  (Felis  pardina),  lion  (Felis  leo), 
hya3na,  elephant,  and  hippopotamus,  the  bones  of  which  have 
been  found  in  the  Pleistocene  deposits. 


Fig.  206. —  Back  view  of  skull 
of  Musk-sheep  (Ovibos  mos- 
chatus,  £),  Brickearth,  Cray- 
ford,  Kent. 


370  GEOLOGY 

After  the  height  of  the  cold  period  or  Ice  Age  had  been 
reached  and  the  general  temperature  of  the  northern  hemisphere 
began  to  rise  again,  the  ice  retreated  from  the  low  grounds, 
but  still  continued  among  the  mountains.  The  existing  snow- 
fields  and  glaciers  of  the  Alps,  the  Pyrenees,  and  Scandinavia 
are  the  lineal  descendants  of  those  vaster  ice-sheets  which  for- 
merly overspread  so  much  of  Europe.  The  glaciers  of  the  Alps, 
large  though  they  are,  can  be  shown  to  be  merely  the  relics  of 
their  former  size.  The  glacier  of  the  Ehone,  for  example,  as  is 
proved  by  rock-striae  and  transported  blocks,  once  extended  170 
miles  in  direct  distance  from  its  modern  termination,  and  rose 
hundreds  of  feet  above  its  present  surface,  burying  the  valleys 
and  overflowing  considerable  ridges  of  hills.  The  glacier  of 
the  Aar  stretched  once  as  far  as  Berne  —  a  distance  of  about 
70  miles  from  its  present  termination;  and,  judging  from  the 
marks  it  has  left  on  the  mountains,  it  must  have  been  not  less 
than  4000  feet  thick  at  the  Lake  of  Brienz. 

Though  elsewhere  in  Europe  the  glaciers  have  long  ago  van- 
ished from  most  of  the  high  grounds,  they  have  left  unmistak- 
able traces  of  their  former  presence.  Thus  in  hundreds  of 
valleys  among  the  Highlands  of  Scotland,  in  the  Lake  District, 
and  North  Wales,  admirably  ice-worn  bosses  of  rock  and  beauti- 
fully perfect  moraines  may  be  seen.  We  can  even  trace,  in 
the  succession  of  moraines  that  become  smaller  as  they  approach 
the  head  of  a  valley,  the  stages  of  retreat  of  the  original  glacier 
as  it  shrank  before  the  increasing  warmth,  till  at  last  it  dis- 
appeared together  with  the  snow-basin  that  fed  it. 

Other  relics  of  the  retirement  of  the  ice-sheet  are  supplied 
by  the  long  mounds  and  heaps  of  gravel  and  sand,  so  abundantly 
strewn  over  many  Lowlands  of  Northern  Europe.  These  some- 
times form  ridges,  rising  20  or  30  feet  above  the  ground  on 
either  side  of  them,  and  running  for  a  number  of  miles.  Else- 
where they  are  heaped  together  irregularly,  often  enclosing  pools 
of  water.  They  are  known  as  Osar  in  Sweden,  Kames  in  Scot- 
land, and  Eslcers  in  Ireland. 

During  the  later  stages  of  the  Ice  Age  the  level  of  the  land 
in  Western  Europe  was  lower  than  it  is  now.  When  elevation 
began,  the  upward  movement  continued  with  long  intervals  of 
rest  until  the  land  reached  its  present  position.  These  pauses 
during  the  prolonged  upheaval  are  marked  by  lines  of  raised 


POST-TERTIARY  PERIODS  371 

beach  which  are  well  seen  along  both  sides  of  Scotland,  and  also 
along  the  sea  margin  of  Norway. 

So  slowly  and  gradually  did  the  great  cold  disappear  that  the 
Ice  Age  insensibly  passed  into  the  Recent  or  existing  period. 
There  can  be  no  doubt  that  man  appeared  in  Europe  before  the 
climate  had  become  as  mild  as  it  now  is,  for  his  flint-flakes 
and  bone  implements  are  found  associated  with  the  bones  of 
Arctic  animals  in  Central  France,  and  traces  of  his  presence 
in  rudely  chipped  stone  instruments  occur  in  deposits  which 
point  to  frozen  rivers.  Indeed,  in  a  certain  sense,  it  may  be 
said  that  the  Ice  Age  still  exists  among  the  snow-fields  and 
glaciers  of  Europe. 

Arranged  in  chronological  order,  the  evidence  from  which 
the  history  of  the  Pleistocene  period  is  determined  may  be  given 
as  follows: — 

Last  traces  of  local  glaciers;  terminal  and  lateral  moraines. 

Marine  terraces  or  raised  beaches,  sometimes  with  moraines  resting 
upon  them ;  rock-shelves  cut  probably  by  waves  and  floating  ice, 
and  marking  former  levels  of  the  sea.  These  beaches  and  shelves 
indicate  pauses  during  the  last  upheaval  of  the  land.  Marine 
clays  with  Arctic  shells. 

Erratic  blocks  chiefly  transported  by  the  great  ice-sheet,  but  partly 
also  by  floating  ice  during  the  rise  of  the  land,  and  by  valley- 
glaciers. 

Sands  and  gravels  (kames)  arranged  in  heaps,  mounds,  and  ridges,  and 
due  in  some  way  to  the  melting  of  the  edges  of  the  ice-sheet,  often 
associated  with  lacustrine  deposits  formed  in  their  hollows,  and 
containing  lake-shells  and  terrestrial  plants  and  animals. 

Boulder-clay,  till,  or  bottom-moraine  of  the  great  ice-sheet;  the  upper 
part  sometimes  rudely  stratified,  and  in  some  regions  separated 
from  the  lower  part  by  a  series  of  "  middle  sands  and  gravels  " ; 
the  lower  part  quite  unstratified  and  full  of  transported  stones 
and  boulders.  Finely  laminated  clays,  sands,  layers  of  peat,  and 
traces  of  terrestrial  surfaces  occur  at  different  levels  in  the  boulder- 
clay,  and  mark  "  interglacial  periods  "  of  milder  climate. 

Polished  and  striated  surfaces  of  rock,  ground  down  by  the  movement 
of  the  ice-sheet. 

RECENT. 

The  insensible  gradation  of  what  is  termed  the  Pleistocene 
into  the  Recent  series  of  deposits  affords  a  good  illustration  of 
the  true  relations  of  the  successive  geological  formations  to  each 


372  GEOLOGY 

other.  We  can  trace  this  gradual  passage  because  it  is  so  recent 
that  there  has  not  yet  been  time  for  those  geological  revolutions, 
which  in  the  past  have  so  often  removed  or  concealed  the  evi- 
dence that  would  otherwise  have  been  available  to  show  that 
one  period  or  group  of  formations  merged  insensibly  into  that 
which  followed  it. 

The  recent  formations  are  those  which  have  been  accumulated 
since  the  present  general  arrangement  of  land  and  sea,  the 
present  distribution  of  climate,  and  the  present  floras  and  faunas 
of  the  globe  were  established.  They  are  particularly  distin- 
guished by  traces  of  the  existence  of  man.  Hence  the  geological 
age  to  which  they  belong  is  spoken  of  as  the  Human  Period. 
But,  as  has  already  been  pointed  out,  there  is  good  evidence  that 
man  had  already  appeared  in  Europe  during  Pleistocene  time, 
so  that  the  discovery  of  human  relics  does  not  afford  certain 
evidence  that  the  deposit  containing  them  belongs  to  the  Kecent 
series.  Nevertheless,  it  is  in  this  series  that  vestiges  of  man 
become  abundant,  and  that  the  proofs  oi  his  advancing  civilisa- 
tion are  contained. 

Man  differs  in  one  notable  respect  from  the  other  mammals 
whose  remains  occur  in  a  fossil  state.  Comparatively  seldom 
are  any  of  his  bones  discovered  as  fossils ;  but  he  has  left  behind 
him  other  more  enduring  monuments  of  his  presence  in  the 
form  of  implements  of  stone,  metal,  bone,  or  horn.  These  relics 
are  in  a  sense  more  valuable  than  his  bones  would  have  been, 
for  while  they  afford  us  certain  testimony  to  his  existence,  they 
give  at  the  same  time  some  indication  of  his  degree  of  civilisa- 
tion and  his  employments.  His  handiwork  thus  comes,  to  possess 
much  geological  value;  his  stone-hatchets,  flint-flakes,  bone- 
needles,  and  other  pieces  of  workmanship  are  to  be  regarded  as 
true  fossils,  from  which  much  regarding  his  early  history  has 
to  be  determined. 

In  the  river-valleys  of  the  north-west  of  France  and  south- 
east of  England  human  implements  have  been  found  in  the 
higher  alluvial  terraces.  After  careful  exploration,  it  has  been 
ascertained  that  these  objects  have  not  been  buried  there  sub- 
sequently, but  must  have  been  covered  up  at  the  time  the  gravel 
was  being  formed.  The  higher  terraces  are  of  course  the  older 
deposits  of  the  rivers,  which  have  since  deepened  their  valleys 
until  they  now  flow  at  a  much  lower  level  (Ch.  Ill) .  The  excava- 


POST-TERTIARY  PERIODS 


373 


tion  of  valleys  must  have  been  a  slow  process.  Within  a  human 
lifetime  it  is  impossible  to  detect  any  appreciable  lowering  of 
the  ground  from  this  cause.  Even  during  the  many  centuries 
of  which  we  have  authentic  human  records,  we  can  hardly  any- 
where detect  proof  of  such  a  change.  How  vast  then  must  have 
been  the  interval  between  the  time  when  the  rivers  flowed  at 
the  level  of  the  upper  terraces  and  the  present  day!  Other 
evidence  of  the  great  age  of  these  higher  alluvia  is  to  be  found 
in  the  number  of  extinct  animals  whose  remains  are  buried  in 
them.  The  human  implements  likewise  bear  their  testimony 
in  support  of  the  antiquity  of  the  terraces,  for  they  are  ex- 


Fig.  207. —  Palaeolithic  Implements,  (a)  Flint  implement,  Reculver  (£), 
chipped  out  of  a  rounded  pebble;  (6)  Flint  implement  (J)  from  old 
river-gravel  at'  Biddenham,  Bedford,  where  remains  of  cave-bear,  rein- 
deer, mammoth,  bison,  hippopotamus,  rhinoceros,  and  other  mammalia 
have  been  found:  (c)  Bone  harpoon-head  (1)  from  the  red  cave-earth 
underlying  the  stalagmite  floor  of  Kent's  Cavern  (a  and  &  reduced  from 
Mr.  Evans's  "Ancient  Stone  Implements"). 

tremely  rude  in  design  and  construction,  indicative  of  a  race 
not  yet  advanced  beyond  the  early  stages  of  barbarism.  In  the 
lower  and  therefore  younger  terraces,  and  in  other  deposits  which 
may  also  be  regarded  as  belonging  to  a  later  date,  the  articles 
of  human  fabrication  exhibit  evidence  of  much  higher  skill 
and  more  tasteful  design,  whence  they  have  been  inferred  to 
be  the  workmanship  of  a  subsequent  period  when  men  had 


374  GEOLOGY 

made  considerable  progress  in  the  arts  of  life.  Accordingly, 
a  classification  has  been  adopted,  based  upon  the  amount  of 
finish  in  the  -stone  weapons  and  implements,  the  ruder  work- 
manship being  assumed  to  mark  the  higher  antiquity.  The 
older  deposits,  with  coarsely  chipped  and  roughly  finished 
human  stone  implements,  are  termed  Paleolithic,  and  the 
younger  deposits  with  more  artistically  finished  works  in  stone, 
bone,  or  metal  are  known  as  Neolithic.  It  will  be  understood 
that  this  arrangement  is  one  rather  for  convenience  of  descrip- 
tion than  for  a  determination  of  true  chronological  sequence. 
It  is  quite  probable,  for  example,  that  some  of  the  palaeolithic 
gravels  date  back  to  the  Pleistocene  Ice  Age,  while  other  deposits 
containing  similar  weapons  and  a  similar  assemblage  of  extinct 
mammals  may  belong  to  a  much  later  time,  when  the  ice  had 
long  retreated  to  the  north.  It  is  obvious,  too,  that  we  know 
nothing  of  the  relative  progress  made  in  the  arts  of  life  by 
the  early  races  of  man.  One  race  may  have  continued  fashioning 
the  paleolithic -type  of  implement  long  after  another  race  had 
already  learnt  to  make  use  of  the  neolithic  type.  Even  at  the 
present  day  we  see  some  barbarous  races  employing  rude  weapons 
of  stone  not  unlike  those  of  the  paleolithic  gravels,  while  others 
fabricate  stone  arrow-heads  and  implements  of  bone  exactly  re- 
sembling those  of  the  neolithic  deposits.  It  would  hardly  be 
incorrect  to  say  that  in  some  respects  certain  tribes  of  mankind 
are  still  in  the  paleolithic  or  neolithic  condition  of  human 
progress. 

i.  PALEOLITHIC. 

The  formations  included  under  this  term  are  distinguished 
by  containing  the  rudest  shapes  of  human  stone  implements, 
associated  with  the  remains  of  mammals,  some  sort  of  which  are 
entirely  extinct,  while  others  have  disappeared  from  the  districts 
where  their  remains  have  been  found.  These  deposits  may  be 
conveniently  classed  under  the  heads  of  alluvium,  brick-earth, 
cavern-beds,  calcareous  tufas,  and  loess. 

Alluvium. —  Reference  has  just  been  made  to  the  upper  river- 
terraces,  which,  rising  sometimes  80  or  100  feet  above  the 
present  level  of  the  rivers,  belong  to  a  very  ancient  period  in 
the  history  of  the  excavation  of  the  valleys,  and  yet  contain  rude 


POST-TERTIARY  PERIODS  375 

human  implements.  The  mammalian  bones,  found  in  the  sands, 
loams,  and  gravels  of  these  terraces,  include  extinct  species, of 
elephant,  rhinoceros,  hippopotamus,  and  other  animals.  The 
human  tools  are  roughly  chipped  pieces  of  flint  or  other  hard 
stone,  and  their  abundance  in  some  river-gravels  has  suggested 
the  belief  that  they  were  employed  when  the  rivers  were  frozen 
over,  for  breaking  the  ice  and  other  operations  connected  with 
fishing.  The  high  river-gravels  of  the  Somme  and  of  the 
valleys  in  the  south-east  of  England  have  been  specially  prolific 
in  these  traces  of  early  man. 

Brick-earths. —  On  gentle  slopes  and  on  plains,  .the  slow 
drifting  action  of  wind  and  rain  transports  the  finer  particles 
of  soil  and  accumulates  them  as  a  superficial  layer  of  loam  or 
brick-earth.  In  the  south-east  of  England,  considerable  tracts 
of  country  have  been  covered  with  a  deposit  of  this  nature. 
It  is  still  in  course  of  accumulation,  but,  as  already  stated 
herein,  its  lower  parts  must  date  back  to  a  high  antiquity, 
for  they  contain  the  bones  of  extinct  mammals,  together  with 
human  implements  of  palaeolithic  type. 

Cave-earth  and  stalagmite. —  The  origin  of  caverns  in  lime- 
stone districts  was  described  in  Chapter  V,  and  reference  was 
made  to  the  formation  of  stalagmite  on  their  floors,  and  to  the 
remarkably  perfect  preservation  of  animal  remains  in  and  be- 
neath that  deposit.  Many  of  these  caves  were  dens  tenanted 
by  hyaenas  or  other  savage  beasts  of  prey.  Some  of  them  were 
inhabited  by  man.  In  certain  cases,  they  have  communicated 
with  the  ground  above  by  openings  in  their  roofs,  through 
which  the  bodies  of  animals  have  fallen  or  been  washed  by 
floods.  The  stalagmite,  by  covering  over  the  bones  left  on  the 
floor  of  the  caverns,  or  in  the  earth  deposited  there  by  water, 
has  preserved  them  as  a  singularly  interesting  record  of  the 
life  of  the  time. 

Calcareous  Tufa. —  Here  and  there,  the  incrustation  of  tufa 
formed  round  the  outflow  of  calcareous  springs  has  preserved 
the  remains  of  the  vegetation  and  of  the  land-animals  of  the 
palaeolithic  time. 

Loess. —  This  is  the  name  given  to  a  remarkable  accumula- 
tion of  pale  yellowish  calcareous  sandy  earth  which  occurs  in 
some  of  the  larger  river  valleys  of  Central  Europe,  especially 
in  those  of  the  Ehine  and  the  Danube;  it  likewise  covers  vast 


376  GEOLOGY 

regions  of  China,  and  is  found  well  developed  in  the  basin  of 
the  Mississippi.  It  is  unstratified  and  tolerably  compact,  so 
that  it  presents  steep  slopes  or  vertical  walls  along  some  parts 
of  the  valleys,  and  can  be  excavated  into  chambers  and  passages. 
In  China  subterranean  villages  have  been  dug  out  of  it,  along 
the  sides  of  the  valleys  which  it  has  filled  up.  It  contains 
remains  of  terrestrial  plants  and  snail-shells,  also  occasional 
bones  of  land-animals.  It  bears  little  or  no  relation  to  the  levels 
of  the  ground,  for  it  crosses  over  from  one  valley  to  another, 
and  even  mounts  up  to  heights  several  thousand  feet  above  the 
sea  and  far  above  the  surrounding  valleys.  Its  origin  has  been 


Fig.  208. —  Antler  of  Reindeer  (^T)   found  at  Bilney  Moor,  East  Dere- 

ham,  Norfolk. 

the  subject  of  much  discussion  among  geologists  and  travellers. 
But  the  result  of  much  careful  investigation  bestowed  upon  it 
goes  to  show  that  the  loess  is  probably  a  sub-aerial  deposit 
formed  by  the  long-continued  drifting  of  fine  dust  by  the  wind. 
It  was  probably  accumulated  during  a  comparatively  dry  period 
when  the  climate  of  Central  Europe,  after  the  disappearance 
of  the  ice-sheet,  resembled  that  of  the  steppes  of  the  south-east 
of  Russia.  The  assemblage  of  animals  whose  bones  have  been 


POST-TERTIARY  PERIODS  377 

found  in  it  closely  resembles  that  of  these  steppes  at  the  present 
time;  for  it  includes  species  of  jerboa,  porcupine,  wild  horses, 
antelopes,  etc.  Among  its  fossils,  however,  there  occur  also  the 
bones  of  the  mammoth,  woolly  rhinoceros,  musk-sheep,  hare, 
wolf,  stoat,  etc.,  together  with  palaeolithic  stone  implements. 

Thus  the  association  of  animals  in  the  palaeolithic  formations 
shows  a  commingling  of  the  denizens  of  warmer  and  colder 
climates,  like  that  already  noticed  as  characteristic  of  the  Ice 
Age,  and  hence  the  inference  above  alluded  to  that  the  paleo- 
lithic gravels  may  themselves  be  interglacial.  Among  the  animals 
distinctively  of  more  southern  type  mention  may  be  made  of 
the  lion,  hyaena,  hippopotamus,  lynx,  leopard,  Caffer  cat;  while 
among  the  northern  forms  are  the  glutton,  Arctic  fox,  reindeer, 
Alpine  hare  (Lepus  varidbilis),  Norwegian  lemming  (My odes 
torquatus),  and  musk-sheep.  The  animals  which  then  roamed 
over  Europe,  but  are  now  wholly  extinct,  included  the  mammoth, 
woolly  rhinoceros,  and  other  species  of  the  genus,  Irish  elk 
(Megaceros  hibernicus),  and  cave-bear  (Ursus  speloeus).  The 
traces  of  man  consist  almost  entirely  of  pieces  of  his  handiwork ; 
only  rarely  are  any  of  his  bones  to  be  seen.  Besides  the  rude 
chipped  flints,  he  has  left  behind  him,  on  tusks  of  the  mammoth 
and  horns  and  bones  of  the  reindeer  and  other  animals,  pre- 
served in  the  stalagmite  of  cavern-floors,  vigorous  incised  out- 
line-sketches and  carvings  representing  the  species  of  animals 
with  which  he  was  familiar,  and  some  of  which  have  long  died 
out.  He  was  evidently  a  hunter  and  fisher,  living  in  caves  and 
rock-shelters,  and  pursuing  with  flint-tipped  arrow  and  javelin 
the  bison,  reindeer,  horse,  mammoth,  rhinoceros,  cave-bear,  and 
other  wild  beasts  of  his  time. 

2.  NEOLITHIC. 

In  this  division,  the  human  implements  indicate  a  consider- 
able advance  in  the  arts  of  life,  and  the  remains  of  the  mam- 
moth, rhinoceros,  and  other  prevalent  extinct  forms  of  the 
palaeolithic  series  are  absent.  The  deposits  here  included  consist 
of  river-gravels,  cave-floors,  peat-bogs,  lake-bottoms,  raised 
beaches,  sand-hills,  pile-dwellings,  shell-mounds,  and  other 
superficial  accumulations  in  which  the  traces  of  human  occupa- 
tion have  been  preserved. 


378 


GEOLOGY 


After  the  extinction  of  the  huge  pachyderms,  the  European 
fauna  assumed  the  general  character  which  it  now  presents,  but 
with  the  presence  of  at  least  one  animal,  the  Irish  elk,  that  has 
since  become  extinct,  and  of  others,  such  as  the  reindeer,  elk, 
wild  ox  or  urus,  grizzly  bear,  brown  bear,  wolf,  wild  boar,  and 
beaver,  which,  though  still  living,  have  long  been  extirpated  from 
many  districts  wherein  they  were  once  plentiful.  This  local  ex- 
tinction has  no  doubt,  in  many  if  not  in  most  cases,  been  the 
result,  directly  or  indirectly,  of  human  interference.  But  man 
not  only  drove  out  or  annihilated  the  old  native  animals.  As 


Fig.  209. —  Neolithic  Implements,  (o)  Stone  axe-head  (J)  ;  (6)  Barbed 
flint  arrow-head  (natural  size)  ;  (c)  Roughly-chipped  flint  celt  (J)  ; 
(d)  Polished  celt  (i),  with  part  of  its  original  wooden  hand  still 
attached,  found  in  a  peat-bog,  Cumberland;  (e)  Bone-needle  (natural 
size),  Swiss  Lake  Dwellings;  a,  6,  c,  d,  reduced  from  Mr.  Evans's 
"  Ancient  Stone  Implements." 

tribe  after  tribe  of  human  population  migrated  into  Europe 
from  some  region  in  Asia,  they  carried  with  them  the  animals 
they  had  domesticated  —  the  hog,  horse,  sheep,  goat,  shorthorn, 
and  dog.  The  remains  of  these  creatures  never  occur  among  the 
palaeolithic  deposits;  they  make  their  appearance  for  the  first 
time  in  the  neolithic  accumulations,  whence  the  inference  has 
been  drawn  that  they  never  formed  part  of  the  aboriginal  fauna 


POST-TERTIARY  PERIODS  379 

of  Europe,  but  were  introduced  by  the  human  races  of  the 
neolithic  period. 

The  stone  articles  of  human  workmanship  found  in  neolithic 
deposits  consist  of  polished  celts  and  other  weapons,  hammers, 
knives,  and  many  other  implements  of  domestic  use.  Knives, 
needles,  pins,  and  other  objects  were  made  out  of  bone  or  horn. 
There  is  evidence  also  that  the  arts  of  spinning,  weaving,  and 
pottery-making  were  not  unknown.  The  discovery  of  several 
kinds  of  grain  shows  that  the  neolithic  folk  were  also  farmers. 
Vast  numbers  of  these  various  relics  have  been  found  at  the  pile- 
dwellings  of  Switzerland  and  other  countries.  For  purposes  of 
security  these  people  were  in  the  habit  of  constructing  their 
wooden  dwellings  in  lakes  on  foundations  of  beams,  wattled- 
work,  stones,  and  earth.  Sometimes  these  erections  were  apt 
to  be  destroyed  by  fire,  as  well  as  to  decay  by  age.  And  their 
places  were  taken  by  new  constructions  of  a  similar  kind  built 
on  their  site.  Hence,  as  generation  after  generation  lived  there, 
all  kinds  of  articles  dropped  into  the  lakes  were  covered  up  in 
the  silt  that  slowly  gathered  on  the  bottom.  And  now,  when 
the  lakes  are  drained,  or  when  their  level  is  lowered  by  pro- 
longed drought,  these  accumulated  droppings  are  laid  open  for 
the  researches  of  antiquaries  and  geologists.  Many  important 
relics  of  neolithic  man  have  likewise  been  obtained  from  the 
floors  of  caverns  and  rock-shelters  —  places  that  from  their  con- 
venience would  continue  to  be  used  as  in  palaeolithic  time.  In- 
teresting evidence,  also,  of  the  successive  stages  of  civilisation 
reached  by  early  man  in  Europe,  is  supplied  by  the  older  Danish 
peat-bogs,  in  the  lower  parts  of  which  remains  of  the  Scotch  fir 
(Pinus  sylvestris),  a  tree  that  had  become  extinct  in  that  country 
before  the  historic  period,  are  associated  with  neolithic  imple- 
ments. In  a  higher  layer  of  the  peat,  trunks  of  common  oak  are 
found,  together  with  bronze  implements,  while  in  the  uppermost 
portion,  the  beech-tree  and  iron  weapons  take  their  place. 

Between  the  neolithic  and  the  present  period  no  line  can  be 
drawn.  They  shade  insensibly  into  each  other,  and  the  ma- 
terials which  reveal  the  history  of  their  geographical  and  climatal 
vicissitudes,  their  changes  of  fauna  and  flora,  and  their  human 
migrations  and  development,  form  a  common  ground  for  the 
labours  of  the  archaeologist,  the  historian,  and  the  geologist. 

During  the  Kecent  period  the  same  agencies  have  been  and 


380  GEOLOGY 

are  at  work  as  those  which  have  been  in  progress  during  the 
vast  succession  of  previous  periods.  In  the  foregoing  pages  we 
have  followed  in  brief  outline  each  of  these  great  periods,  and 
after  this  survey  we  are  led  back  again  to  the  world  of  to-day 
with  which  the  first  chapters  of  this  book  began.  In  this  circle 
of  observation  no  trace  can  anywhere  be  detected  of  a  break  in 
the  continuity  of  the  evolution  through  which  our  globe  has 
passed.  Everywhere  in  the  rocks  beneath  our  feet,  as  on  the 
surface  of  the  earth,  we  see  proofs  of  the  operations  of  the  same 
laws  and  the  working  of  the  same  processes. 

Such,  however,  have  been  the  disturbances  of  the  terrestrial 
crust  that,  although  undoubtedly  there  has  been  no  general 
interruption  of  the  Geological  Eecord,  local  interruptions  have 
almost  everywhere  taken  place.  The  sea-floor  of  one  period  has 
been  raised  into  the  dry  land  of  another,  and  again,  the  dry- 
land, with  its  chronicles  of  river  and  lake,  has  been  submerged 
beneath  the  se^.  Each  hill  and  ridge  thus  comes  to  possess  its 
own  special  history,  which  it  will  readily  reveal  if  questioned 
in  the  right  way. 

We  are  surrounded  with  monuments  of  the  geological  past. 
But  these  monuments  are  being  slowly  destroyed  by  the  very 
same  processes  to  which  they  owed  their  origin.  Air,  rain, 
frost,  springs,  rivers,  glaciers,  waves,  and  all  the  other  con- 
nected agents  of  demolition,  are  ceaselessly  at  work  wherever 
land  rises  above  the  sea.  It  is  in  the  course  of  this  demolition 
that  the  characteristic  features  of  the  scenery  of  the  land  are 
carved  out.  The  higher  and  harder  parts  are  left  as  mountains 
and  hills,  the  softer  parts  are  hollowed  out  into  valleys,  and  the 
materials  worn  away  from  them  are  strewn  over  plains.  And 
as  it  is  now,  so  doubtless  has  it  been  through  the  long  ages  of 
geological  history.  Decay  and  renovation  in  never-ending  cycles 
have  followed  each  other  since  the  beginning  of  time. 

But  amid  these  cycles  there  has  been  a  marvellous  upward 
progress  of  organic  being.  It  is  undoubtedly  the  greatest  tri- 
umph of  geological  science  to  have  demonstrated  that  the  present 
plants  and  animals  of  the  globe  were  not  the  first  inhabitants 
of  the  earth,  but  that  they  have  appeared  only  as  the  de- 
scendants of  a  vast  ancestr}7", —  as  the  latest  comers  in  a  majestic 
procession  which  has  been  marching  through  an  unknown  series 
of  ages.  At  the  head  of  this  procession  we  ourselves  stand^ 


POST-TERTIARY  PERIODS  381 

heirs  of  all  the  progress  of  the  past  and  moving  forward  into  the 
future  wherein  progress  towards  something  higher  and  nobler 
must  still  be  for  us,  as  it  has  been  for  all  creation,  the  guid- 
ing law. 

THE  END. 


INDEX. 


Accumulations  formed  by  the  sea, 

77 

•Adelsberg  caverns,  55 
Age  of  reptiles,  the  Jurassic,  316 
Albian  stage  of  the  Jurassic  period, 

334 

Alluvium,  how  formed,  35 
Aluminium,   125 
Ammonites   in   the   triassic   period, 

305 

in  the  Jurassic  period,  313 
Amphibians    of    the    carboniferous 

period,  284,  286 
Amphibolites,  176 
Anchor-ice,  63 
Anhydrite,  141 
Animals    as    causes    of    change    on 

earth's  surface,  18 
durable  parts  of,  223 
geological  records  made  by, 

83-95 
deposits  formed  by  remains  of, 

87 

remains  in  sedimentary  depos- 
its, 91 

of  the  tertiary  periods,  339 
Annelids  in  cambrian  period,  253 
Apatile,   141 
Aragonite,    140 
Archaean  rocks,  242 
Archaeopteryx,  318,  319 
Atlantosaurus,  318 
Atmosphere,       influence       of       on 

changes    of   earth's    surface, 

10-23 

Augite,   138 
Avalanches,  64 
Axmouth  landslip,  52 

Bajocian  stage  of  Jurassic  period, 

321 
Bathonian    stage    of    the    Jurassic 

period,  321 
Barytes,  141 
Barium,   127 
Basalt-rocks,  171 
Birds  first  appear  in   the  Jurassic 

period,  318,  319 
in  the  cretaceous  period,  332 
of  the  eocene  period,  342 


383 


Blow-holes,  73 

Bosses,  in  eruptive  rocks,  209 

Bone-beds,   167 

Boulders  transported  by  ice,  63 

Brachiopods    of    the    devonian    pe- 
riod, 274 
in    the    carboniferous    period, 

290 

of  the  permian  period,  298 
of   the   Silurian   period,   265 

Breccia,  155 

Brick-earth,   20 

Brontosaurus,  318 

Cainozoic     rocks.       See     Tertiary 

Periods. 

Calcareous  springs,  57 
Calcite,  139 
Calcium,   125,   127 
Calcareous    tufa,   59 
Gale-sinter,  59 
Cambrian    period,    246-257 
Carbon,  122 
Carbonates,  139 
Carboniferous  period,  277-294 
Caverns  formed  by  springs,  55 
Cenomian    stage    of    the     Jurassic 

period,  334 
Cephalopods    in    cambrian    period, 

256 
in    the    carboniferous    period, 

291 
characteristic    of    the    triassic 

period,   305 

in  the  cretaceous  period,  329 
in  the  cretaceous  period,  330 
Chalk,   164 

Chalybeate  springs,   61 
Chemical  action  of  running  water, 

25-27 

Chemical  action  of  springs,  54 
Chlorides,  142 
Chloride-schist,   176 
Chlorine,   124 
Chlorite,    138 
Concretions,  143 
Chronology,    geographic    shown    by 

fossils,  228 
Clay,  158 


384 


INDEX 


Clay-slate,  175 

Cleavage  of   rocks,  201 

Cliff-debris,   154 

Climatic  changes  shown  by  fossils, 

227 

Coal,    165 

Coal   seams   as   indicative   of   sub- 
sidence,  279,   280 
Coal,  how  formed,  280 
Concretions,  186 
Concretionary   rocks,   147 
Conglomerate,    156 
Conifers    in    the    permian    period, 

298 

Consolidation    of    sedimentary    de- 
posits, 193 
Continents      formed      in      tertiary 

periods,  339 

Coral-reefs,  how  formed,  89-90 
Coral-rock,  164 

Corals  of  Silurian  period,  261 
of  the  devonian  period,  273 
of  the  devonian  period,  275 
in    the    carboniferous    period, 

287 

in   the  j"^assic   period,   311 
Corralian    stage    of    the    Jurassic 

period,  322 

Cretaceous   period,    324-337 
Crinoids  or  sea-lilies,  253 
of  silurian  period,  261 
in    the    carboniferous    period, 

288 

in  the  triassic,  305 
in  the  Jurassic  period,  311 
Crustaceans  of  the  devonian  period, 

272 
in    the    carboniferous    period, 

289 

in  the  Jurassic  period,  314 
Crystal  forms,  129-131 
Crystalline,  148 
Cteosaurus,  317 
Curvature  of  strata,   198 
Cuttle  fish  in  the  Jurassic  period, 

313 

Cycads  in  the  permian  period,  298 
in  the  Jurassic  period,  310 
age  of,  304 

Danian  stage  of  the  Jurassic  period, 

336 

Dead  Sea,  50 
Dendrites,  135 
Deposition  by  running  water,  35- 

43 

Deposition  by  springs,  56 
Devonian   period,   268-276 
Diabase,   173 
Diatoms,   86 
Dinosaurs  first  appear  in  the  trias- 

sio    period,   307 
in  the  Jurassic  period,  317 


Diorite,  173 

Dip  of  strata,  195 

Dislocation  of  strata,  203 

Dolomite,  140,  161 

Dust,  changes  wrought  by,  20 

Earth,  hot  interior  of,  7 

interior,  condition  of,  97 
crust,  structure  of,  179-234 

Earthquakes,   114-118 

Earthworms,  effect  of  on  soil,  18 

Echinodermata,    in    Cambrian    pe- 
riod, 253 
of  silurian  period,  262 

Elements  and  minerals,  119 

Erosion,  30-35 

Erosion,  geological  effect  of,  G8 

Erosive  power  of  running  water,  25 

Erratic    boulders,    66 

Eruptive   rocks,    148 

Eruptive    rocks,    167-180 

Eruptive   rocks,   208 

Eocene  period  of  the  tertiary,  341- 
346 

Eophyton,  251 

Eozoon,  243 

Fairy  stones,  143 

Faults  in  strata,  203 

Fauna  of  silurian  period,  259 
of  the  permian,  297 
of  the  cretaceous  period,  324 

Felsite,  169 

Felspars,   136 

Ferns  of  the  carboniferous  period, 
282 

Fire-clay,  158 

Fishes  of  the  silurian  period,  265 
of  the  devonian  period, 
in    the    carboniferous    period, 

291 

of  the  permian  period,  299 
of  the  triassic  period,  306 
in  the  Jurassic  period,  315 
of   the   cretaceous   period,   331 

Flint,  106 

Flood-plain  of  rivers,  40 

Flora  of  the  triassic  period,  304 
of  the  Jurassic  period,  310 

Flourine,    125 

Flourides,  142 

Foot-prints,  184 

Forests,    submerged,    11C 

Fragmental   rocks,    146 

Fresh-water  lakes,  how  formed,  44 

Frost,   changes   on   earth's   surface 
by,  13 

Fossil    remains,   220 

Fossilisation,  223 

Fossils,     how     indicative     of    geo- 
graphic change,  226 


INDEX 


385 


Gasteropods    in    cambrian    period, 

255 

of  the  silurian  period,  264 
in    the    carboniferous    period, 
291 

Geographic    changes    indicated    by 
fossils,  220 

Geologic     chronology,     shown     by 
fossils,   228 

Geological  record,  table  of,  240,  241 

Geology,  nature  of  its  inquiries,  5 
the  study  of  earth's  history,  9 

Glacial  period,  364 

Glaciers  and  ice-sheets,  65-71 

Glassy   rocks,   149 

Glen   Roy,  parallel  roads  of,  46 

Globe,  earliest  condition  of,  234 

Globigerina   ooze,  88 

Gneiss,  176 

Granite,   170 

Gravel,    155 

Graywacke,  157 

Great  Salt  Lake,  50 

Greenland,   fossils  of,  326,  327 

Ground-ice,  63 

Guano,   166 

Gypsum   in   lakes,   50 

Gypsum,   140,    162 

Hematite  or  specular  iron,  133 
Hornblende  or  amphibole,  137 
Horse,  ancestors  of  appear  in  the 

eocene  period,  344 
Hydrogen,  124 

Hydrozoa  of  silurian  period,  260 
Hydrozoon,   252 

Ice-records,    63-71 

Ice    of    rivers    and    lakes,    changes 

wrought  by,  63 
Iceland   spar,    128 
Ichthyosaurus  or  sea-lizard,  316 
Iguanodon,    last    of    the   dinosaurs, 

331 

Infusorial  earth,  85 
Insects  of  the  carboniferous  period, 

282,  285 

Iron  deposits  in   lakes,  49 
Iron,  127 
Ironstone,    163,    166 

Joints  in  sedimentary  rocks,  193 
Jurassic  period,  310-323 

Kaolin,  158 

Kimmeridgian   stage   of  the   Juras- 
sic period,  322 

Lamellibranchs,    in    cambrian    pe- 
riod,  255 
in    the    carboniferous    period, 

290 

in  the  triassic  period.  305 
in  the  Jurassic  period,  313 


Lakes,  memorials  left  by,  44-50 

filling  up  of,  45 
Lake  deposits,  47 
Land,  surface  of,  changing,  1-5 

demolished  by   the   sea,  72 
Landslips,   52 
Lava,  varieties  of,  100 
Lias   rocks,  320 
Lignite  or  brown  coal,   165 
Limestone,    55,    160,    164 
Limestone     in     the     carboniferous 

period,  278 

Limonite,  or   brown\  iron  ore,   134 
Lizards    first    appear    in    the    per- 

mian  period,  300 

Lizards  in  the  triassic  period,  3o6 
Loess,    159 
Loess,   375 
London,    Roman    remains    in,    3 

Magnesium,  126 

Magnetite,  134 

Mammals  first  appear  in  the  trias- 
sic period,  307 
of  the  eocene  period,  343 
in  the  Jurassic  period,  319 

Mammoth,  369 

Mammoth  Cave,  56 

Manganese,   127 

Manganese  oxides,   135 

Mangrove-swamps,  85 

Marble,  177 

Marl  in  lakes,  48 

Marcasite,   143 

Mastodon,  352 

Mechanical      action      of      running 

water,  27-30 
of  springs,  51 

Megalosaurus,  317 

Metals,  120,   125 

Metamorphism   in  geology,  206 

Metaloids,  120 

Mesozoic   periods,  302-337 

Mica,  137 

Mica-schist,    176 

Minerals  and  elements,    119 

Minerals,  importance  of  in  earth's 
crust,    128-144 

Mineral  veins,  215 

Miocene  period,  350^-356 

Mollusca    of    the    silurian    period, 

264 

of  the  devonian  period,  275 
in    the    carboniferous    period. 

285 
of  the  eocene  period,  342 

Moraines,   how   formed,  65 

Mosasaurus,    332 

Mylonitic   rocks,   152 

Nature  and  use  of  fossils,  220,  233 
Nebular   hypothesis  stated,  235 
Necks,  or  volcanic  vents,  215 


386 


INDEX 


Neocomian  stage  of  the  Jurassic 
period,  334 

Neolithic  period,  377 

Niagara  Falls,  show  power  of  ero- 
sion, 34 

Nitrogen,  125 

Obsidian,  168 

Old     red     sandstone     period.     See 

Devonian   Period. 
Oligocene  period,   346-349 
Olivine,  and  serpentine  rocks,  173 
Olivine,  138 
Oolitic  rocks,  148 
Ooze,   88 
Organic    remains,    how    preserved, 

221 

Orthoclase,  136 
Orthoclase  rocks,  168 
Outcrop  of  strata,  197 
Overlap,  in  geology,  190 
Oxfordian    stage    of    the    Jurassic 

period,  321 

Oxides,  abundance  of,  132 
Oxygen,  importance  of,  121 
Oysters    abuMant    in   the   Jurassic 

period,  313 

Palaeolithic  period,  374 

Palaeozoic  periods,  246-301 

Peat,  165 

Peat-bogs,  84 

Peak  cavern,  55 

Peridot,   138 

Perlitic  rocks,  150 

Permian  rocks,  how   formed,   295- 

301 
Petrifaction    of    organic    structure, 

225 

Petrifying  springs,  60 
Phosphorus,  124 
Phosphates,  141 
Phyllopod  crustaceans  of  the  silu- 

rian  period,  263 
Pines  appear  in  the  Jurassic  period, 

311 

Plagioclase,   136 
Plants,  geological  records  made  by, 

83-95 

durable  parts  of,  222 
of  the  devonian  period,  269 
of    the    carboniferous    period, 

282-284 

of  the  permian  period,  297 
of  the  cretaceous  period,  325 
of  the  tertiary  period,  339 
of  the  eocene  period,  341 
Plesiosaurs,  first  appear  in  the  tri- 

assic  period,  307 
Plesiosaurus,  316 
Pliocene   period,   356-362 
Pleistocene  group,  363 
Polyzoa   in   the  carboniferous  per- 
iod, 289 


Portlandian   stage  of  the  juraasic 

period,  322 

Post-tertiary    periods,    363-381 
Potassium,   126 
Pot-holes,  how  made,  30 
Porphyritic  rocks,  150 
Pre-cambrian  period,  241,  242 
Pterodactyl,   317 
Purberkian    stage    of    the   Jurassic 

period,  322 
Pyrite,   143 
Pyroxene,   138 

Quarries,  geological  evidence  of,  4 
Quartz  crystals,  122,  132 
Quartzite,  177 

Rain,  a  cause  of  change  on  earth's 

surface,  14,  18,  20 
Rain-prints,  184 
Raised   beaches,   116 
Recent  period,  371 
Reptiles  in  the  triassic  period,  306 
development    of    characteristic 

of  the  Jurassic  period,  31(5 
of  the  cretaceous   period,   331 
River-action,  records  of,  35 
River   deltas,    78 
River-terraces,   39 
Rock-salt  in  lakes,  50 
Rock-salt,  163 

Rocks,  geological  evidence  of,    119 
Roman  remains  in  London,  3 
Rotation    of    earth    in    early    ages, 

237 
Running      water,      in      geological 

changes,  24-43 
Ripple  marks,  183 

Salt-lakes,  formation  of,  49 

Sand,  156 

Sand  dunes,  changes  of,  21 

Sandstone,  157 

Saturation    and   dessication,    influ- 
ence of,    13 

Schistose  grit,  177 

Schistose  rocks,  174 

Sea,    geological    records    made    by, 
72-82 

Sea-serpents  of  the  cretaceous  pe- 
riod,  332 

Sea-weeds,  accumulations  of,  86 
as  land  makers,  87 

Secondary    era,    see    Mesozoic    Pe- 
riods. 

Sedimentary  rocks,   153,   159,  146, 
179-207 

Senonian     stage    of    the    Jurassic 
period,  336 

Serpentine,  139 

Serpentine   and   olivine   rocks,    173 

Shale,  158 

Shearing  of  strata,  199 


INDEX 


387 


Sheets,  of  eruptive  rocks,  211 

Shell-marl,  164 

Shell-banks,   87 

Siderite,  146 

Siliceous  springs,  61 

Silicates,  135 

Silicon,  importance  of,  122 

Silurian  period,  258-268 

Snow,  geological  effects  of,  64 

Sodium,    126 

Soil  beneath  cities,  nature  of,  3 

Soil  and  subsoil,  changes  in,  16-22 

Sphagnum,  or  bog  moss,  growth  of, 
85 

Spherulitic  rocks,  150 

Springs,  geological  records  of,  51- 
62 

Stalactites,  how  formed,  57 

Stalagmites,  how  formed,  57 

Stegosaurus,  318 

Stone     implements     found     under 
London,  3 

Stones  transported  by  rivers,  38 

Storm  beaches,   79 

Stratification,    180 

Strike  of  strata,   196 

Strata,   alternation   of,   187 

chronological    value    of,   188 
conformability  of,  190 

Subsidence  of  land,  278-280 

Subsidence,  proofs  of,  189 

Sulphates,  140 

Sulphides,  143 

Sulphur,  124 

Sun-cracks,  184 

Syenite,   169 


Temperature,  influence  of  changes 
in,  13 

Tertiary  periods,  338-362 

Titanic  iron,  135 

Trachyte,   169 

Transporting  power  of  running 
water,  25 

Transportation  by  glaciers,  65 

Travertine,  59 

Trilobites  in  cambrian  period,  254 
of  the  silurian  period,  263 
in    the    carboniferous    period. 

288 
of  the  devonian  period,  273 

Triassic  period,  302-310 

Turonian  stage  of  the  Jurassic  pe- 
riod, 335 

Upheaval  and  subsidence,  115 

Veins  and  dykes,  214 

Vesicular  rocks,  151 

Volcanic  ash,  159 
dykes,  112 
products,  100-107 
vents  and  fissures,  107 

Volcanoes,  importance  of  their 
study,  7 

Volcanoes  and  earthquakes,  96-114 

Water,  running,  in  geological 
changes,  24-43 

Weathering,   influence   of,   11-14 

Wind,  effect  of  on  earth's  sur- 
face, 18 

Winding  of  streams,  how  formed. 
32 


Talus,  how  formed,  19 


Zeolites,  136 


